Sure, you can relax in the Florida Keys. The southern sun warms, the breezes soothe and you soon lapse into a tropical languor that is disturbed only by the insistent urge for another frosty pina colada. That's one kind of Keys.

The other kind is equally tempting, but it means rousing yourself from your beach blanket and engaging in the water and other outdoor sports that are abundantly available almost anywhere you stay on this remarkable chain of islands.

Spring is a perfect time to visit the Keys. The big winter crowds of its busiest season have gone home; the weather is no longer subject to winter's occasional cold snaps; and lodging prices drop considerably. You also can avoid the college revelers on Spring Break, who mostly stick to the beaches of Fort Lauderdale and beyond. In fact, the Keys at this time of year are where Floridians go for their weekend getaways.

To most outsiders, the Keys mean Key West, the historic and sophisticated city at the end of the island chain. This is a story about the rest of the Keys -- the ones you cross before you get to Key West.

The Florida Keys are actually hundreds of islands, topping an ancient coral reef that once extended southwest into the Gulf of Mexico from the tip of the Florida peninsula. Among the largest is the resort island of Key Largo, about 30 miles long and two miles wide. The most populated is Key West, with about half of the Keys' 63,000 residents.

Many other islands are too tiny -- no more than a spot in the sea -- or too swampy to be inhabited. Only a few can be reached by the only connecting road, the famous 126-mile Overseas Highway from the Miami area to Key West.

Heading down the highway, you hop from island to island, crossing 42 bridges, including one that is seven miles long. On the left are the inviting waters of the Atlantic Ocean; on the right, the Gulf of Mexico, both an iridescent jade in the morning changing to deep blue in late afternoon.

* The sea's proximity -- in any direction -- has made the Keys a year-round center for deep-sea and other game fishing; you can rent a charter yacht for $425 a day or drop a line from one of the bridges for only the cost of live bait -- $1.25 for a dozen shrimp. The Keys also are a national capital for snorkeling, scuba diving, sailing and sailboarding.sw sk

The locals will tell you the islands offer the best conditions in the country for all of these sports -- which is why many have moved there from "up north." And they can make a pretty strong case for their contention.

Chiefly, the reason is because the country's only living coral reef system parallels the Atlantic side of the Keys just a few miles offshore. The reef, harboring more than 600 varieties of tropical fish and 40 kinds of coral, is the big lure for snorkelers and divers. Expert divers will find challenges, and qualified instructors teach the beginners and guide them on underwater tours. If you don't dive, you still can easily see the reef as you snorkel on the surface because the water is so clear.

There is one important fact about which any sunbather going to the Keys should be forewarned: For all their attributes, the Keys are very short of good, sandy beaches, though most resorts offer at least a small patch of sand on which to spread your towel.

Many of the uninhabited Keys are wilderness areas reached only by boat. They and some of the less-populated islands beckon nature enthusiasts. These wilderness areas can be explored by foot, bicycle, canoe, raft or car.

At the 14,000-acre National Key Deer Refuge on Big Pine Key, we drove back roads seeking a glimpse of the tiny white-tailed deer, a poodle-sized version of their larger northern cousins. The deer, an endangered species numbering about 300, eluded us, but when we stopped to walk around Blue Pond, a fresh-water quarry near refuge headquarters, we all but stepped on a giant alligator sunning himself on a rock. A refuge sign, and one to be heeded, warns against swimming.

One afternoon, I climbed into a motor-powered rubber raft for a fascinating two-hour tour around Tom's Harbor Keys, two adjacent mangrove islands in the middle Keys that are havens for all kinds of seabirds, including the stately great white heron. We beached the boat and waded along the shore line searching for sea urchins, sea cucumbers, sponges, coral and other underwater life -- only to observe, not to take. But our attention was soon drawn by a mother osprey, guarding her nest high atop a mangrove tree.

The nest was a huge one, at least a foot and a half in diameter. As we approached, stepping around a lobster trap blown ashore by a storm, she chirped a noisy warning, and her mate, circling overhead, soon joined in. Neither quieted until we had passed well away to resume our study of the islands' tidal pools. This outing was arranged by our hotel, Hawk's Cay on Duck Key -- about halfway down the island chain -- and other tour outfitters in the Keys offer similar nature-study tours.

From Key Largo south, the highway, U.S. Route 1, is lined with lodgings that range from luxury resorts such as Hawk's Cay ($95 a night and up for two, beginning in April), to many state and private campgrounds that charge only a few dollars a night. You can stay practically anywhere to sample the many water activities.

One of the best of the few beaches on the Keys is at the Bahia Honda State Recreation Area on Bahia Honda Key at the southern end of Seven-Mile Bridge. Elsewhere, a sunbather may be disappointed. It's almost as if nature were telling you something: Get up and get into (or onto) the water.

I'm no fisherman, but Hawk's Cay offers its guests a free two-hour "starter" lesson for beginners from its marina pier, so I signed up -- mostly to find out what everybody along the bridges and out in their boats was catching.

Tim Knight introduced himself as my instructor, a 27-year-old former Navy diver who does most of his fishing under water, supplying live tropical fish to municipal aquariums. "There isn't any better fishing in the United States," he said. And the best time for it is spring through summer. My lesson began with the basics, which meant learning how to hook the squirming bait -- live shrimp -- and cast it into the water without snarling the reel. The first I managed a bit reluctantly; the second I fumbled, and while Knight attempted to untangle the havoc I'd wrought, he gave me a verbal tour of the variety of Keys fishing.

Certainly the least expensive way to go fishing is from one of the old, now-abandoned bridges along the route to Key West, which have played an important role in the Keys. The foundation of the Overseas Highway was a rail line opened to Key West in 1912, linking the Keys to the mainland and opening the islands to tourism.

Disaster struck on Labor Day in 1935 when a massive hurricane destroyed the railway. In its place, the highway was constructed over the railbed, opening in 1938. In recent years, many of the original bridges have been replaced -- but not torn down. They have been reserved for fishing, and vehicle traffic is prohibited.

Since some of the bridges are quite long, many enthusiasts wheel their gear, lunch and refreshments out in shopping carts or wagons, setting up chairs and a table -- and occasionally a tent for shade. "It puts you out in the middle of the ocean," said Knight, "and you don't have to rent a boat."

An advantage over ocean-going boats is that "you don't get seasick," he added, and you can quit and go home when you feel like it. Normally plentiful catches include the good-eating grouper and snapper and even huge tarpon at 100 pounds plus that will give you and your gear a workout. No license is required for saltwater fishing.

Well-heeled fishermen can charter an ocean-going motor launch from the many marinas that line the highway. The 46-foot Huntress II out of Duck Key charges $425 for an eight-hour day on the Atlantic, which can be split among four to six people. The catch is mako shark, the elegant sailfish, blue marlin, tuna and barracuda -- the big fish that become mounted trophies. So-called "party" boats, charging about $20 a person for larger groups, will give you a taste of the sport.

"Backcountry" is another type of fishing, and one that has given the Keys its international reputation for first-class sport. Backcountry generally refers to fishing in the shallow flats on the Gulf side of the Keys. According to Knight, experienced fishermen often switch to backcountry once they have hooked their deep-sea trophy because it offers more challenge.

On the deep-sea boats, the captain and crew assist in the catch; on a backcountry boat it's mostly up to you. A boat and guide, available at marinas such as Duck Key, costs about $185 a day. The prize is bonefish, a six- to 12-pound feisty fighter that is considered one of Florida's favorite gamefish.

For about an hour, Knight helped me cast my line and reel it in -- always empty, though sometimes, he said, beginners fishing from the same pier manage to get a strike every time their bait hits the water. Well, there is some relaxation in casting and reeling, I found -- a consolation of sorts -- and in time, the tangles came less frequently. At least I would not shame myself before the gang at the bridge.

A drive down the Overseas Highway, unfortunately, is not always the scenic spectacle you might hope for. Especially at its northern end on Key Largo, U.S. Route 1 is a frequently unsightly jumble of tourist services, shopping centers and gaudy billboards. But don't let the clutter put you off. The beauty -- and it builds as you get farther south -- is in the really magnificent open views of the sea and islands from the 42 bridges. You're always only short minutes from the next bridge and the next great seascape.

Along the way, keep your eyes open for the discreet green highway markers counting each mile from Florida City, just south of Miami, to Key West. Southbound, the count begins at 126 and decreases to zero. The markers are important guides, since many resorts, motels and other tourist attractions use them for directions. For example, John Pennekamp Coral Reef State Park is listed at Mile Marker 102.5 on Key Largo -- a recommended first stop that will give you a good introduction to the Keys.

The park is sort of a sampler of all of the best of the outdoor Keys: There are woodland trails through mangrove swamps, which give a good idea of what all the Keys once looked like; three sandy swimming beaches on the Atlantic shore, warmed by the Gulf Stream; and a 21-mile-long stretch of offshore coral reef, which you can see in several ways.

Most visitors explore the reef aboard one of the glass-bottom boats departing from the park marina daily at 9 a.m., noon and 3 p.m. for a two-hour cruise (adults, $8.50; under 12, $4). The reef is as colorful as its tropical inhabitants, who swarm beneath the ship. Sports-minded vacationers can join one of the frequent guided tours for snorkeling, scuba diving (certified divers only) and sailing aboard a six-passenger trimaran.

Inside the visitor center, interesting displays detail the natural history of the Keys; there's a replica of a tidal pool where you can carefully pick up some of the curious wiggly inhabitants; and a large aquarium recreates a coral reef alive with swimming and crawling creatures of the deep.

Pay attention to the center's exhibits because they provide interesting insights. The red mangroves -- a large bushlike tree that you see everywhere -- are, for example, very important to the ecology of the Keys. Ringing the shoreline, their mass of twisted, tubular roots standing in saltwater provide erosion protection against storm-whipped waves, and their dead leaves feed many of the creatures that become food for larger fish. They are, of necessity, a salt water-tolerant tree, expiring the salt through their leaves.

This biology lesson should help you appreciate the mangrove; otherwise, it is easily dismissed as a rather scraggily plant sometimes blocking a good view. For mangroves as well as people, a pretty face isn't everything.

We ended our drive south at Mile Marker 61, on tiny Duck Key, a five-island cluster of luxury vacation homes and Hawk's Cay, one of finest resorts in the Keys. "Cay," pronounced "Kay," comes from the Spanish word for a small island, cayo.

The resort is a gem, a 60-acre, flower-draped island retreat that has the tropical flavor of a West Indies inn, plus easy access to a full range of fishing and water sports as well as tennis. Guests can even take a swim and a ride with the half-dozen or so trained dolphins who live in a fenced lagoon just beyond the oceanside swimming pool.

You don't exactly climb aboard a dolphin as you would a horse; you grasp a fin and it pulls you, like a flash, across the water. When the dolphin dives, you let go. The dolphins earn their keep at summer sea shows around the country, wintering at Hawk's Cay, where they put on a short performance at feeding time three times daily. They share the lagoon with two sea lions, Brownie and Claudia, who are a part of the free show.

The inn, built in 1959, previously was known as the Indies Resort. The Indies was sold in 1983, and the new owners invested $8 million in attractive renovations and reopened under the new name in January of 1984. It is a sprawling low-rise complex with lots of open windows to catch the breezes and features a sandy-bottomed swimming lagoon, a pair of outdoor hot tubs and a large fishing marina, where you can dock your own yacht or charter one.

Breakfast at Hawk's Cay, included in the room price, is treated with great respect -- it's a fine buffet served in a gorgeous garden dining room. A chef prepares Belgian waffles, eggs as you like them and other specialities while you wait, and you can pour your own fresh orange juice right from the squeezing machine. TV newsman Roger Mudd stood in the buffet line in front of me earlier this month.

And, as I did, guests can sign up for a free two-hour fishing lesson, or take the beginner's course in sailing and snorkeling. They keep you quite active at Hawk's Cay, if you are willing, but that really is the best way to get to know the Keys.