In the West Indies, where sailing is the sport, yachtmen have a little code to go by: Blue water's fine, green water is less fine and when the water's brown turn around.

It's not really brown water at all, but the shadow of a coral reef poking through the crystal clear depths. It spells danger to yachties, who don't fancy holes beneath the waterline. To another brand of sportsman it spells paradise.

Nowhere is there a more majestic vista for skindivers than a living coral reef. The American continent has only one. It begins off the Florida coast just north of Miami and extends south as far as the Dry Tortugas.

In years past the reef was plundered by trophy seekers and commercial sellers of coral curios. Almost two decades ago the state and federal governments combined in an unprecedented venture to save a big section of the living reef.

The result of their joint effort was John Pennekamp State Park, the nation's first underwater sanctuary. It lies six miles off the Florida Keys and is today a diver's dream.

We were five aboard the Mary's Cary, Capt. Mary Emmott at the helm, when we left Pennekamp last week for an afternoon dive in the reef. We had each paid our $13, which entitled us to snorkel, mask and fins, roundtrip boat ride and more than an hour of underwater exploring.

These is no better bargin anywhere.

"It's not real clear today but there's 30-feet visibility, which is good enough," said Emmott as she anchored up after the half-hour run across the sandy flats.

Off the bow the water shifted sharply from a bright green to an angry brown, and the tops of elkhorn coral poked through the chop.

Any barracudas out there?

"Oh, you'll see plenty of them, but don't worry," said Emmott. "They've seen thousands of divers over the years and they haven't bothered any yet.

"Just don't go sticking your hands in the caverns if you don't know what's inside 'em. There's a lot of moray eels around and I don't feel like making any rush trips to the hospital.

"And remember, this is a sanctuary. You break off one piece of coral and you're destroying 1,000 years of growth. Leave it like you found it."

Then we were overboard, slurping and coughing into the unfamiliar snorkels.

It was a 50-yard, one-minute swim to where the coral outcroppings began at a place the locals call Grecian Rocks. It was the last any of us measured distance or time until the expedition came to an end with a toot on Emmott's boat horn.

The reef is a place of almost indescribable fascination. It teems with such a profusion of life that after a while Mitch and I, overwhelmed, decided to stop at one spot and just stare. "We're going too fast," he burbled through his snorkel tube. "The longer I look the more I see."

I couldn't have agreed more. The reef is an underwater city as busy as K Street at rush hour, only here the commuters are neon gobis, triggerfish, parrotfish, barracuda, yellowtail, crevalle jacks, sergeant majors, porcupinefish and hundreds of other species.

It's an array of incredible color, the likes of which the neon-mongers at Las Vegas could only dream of duplicating.

Why, Mitch wondered, are tropical fish so colorful?

"If you knew that," said Dr. Gilbert Voss, a biologist and reef fiend who hatched the idea for the underwater state park back in the early '60s, "you'd know something all of us have always wanted to know."

There are some theories, and they show in their own little way how wild and complicated the reef culture is.

Bright fish, said Voss, are thought by some to wear their extraordinary colors as a warning to predator fish that they are inedible -- poisonous or spiny.

But, he added, that works in other ways too. Some perfectly edible small fish have evolved and prospered on the reef because they duplicate the inedible fishes' colors. Neat.

There are other types of fish that carry bands of color. Many of them serve as "cleaning fish." They pick parasites off larger predators, who in turn allow the litle fellows to sidle up and nibble at their skins.

But then there are some little banded fish that don't have that in mind at all. "They sidle up to the predators," said Voss, "and instead of nibbling off the parasites they take a chunk of meat out of the big fish's side."

The Florida reef is thought to contain more different species than practically any other; almost 600 species have been counted. The broad spectrum arises because Florida lies at a great aquarian meeting ground for fauna from both island and deep sea habitat, according to Voss.

For whatever reason, it's a remarkable place.

Mitch and I lay still and watched a school of small bait fish being swept in and out of a coral crevice by the tide. A barracuda lay nearby, hanging in limbo with its toothy mouth agape.

Whoosh! The 'cuda swept in and snatched a fish from the school. Yet nothing changed among the little fish. The school stayed where it was and the 'cuda went back to its lair.

Death on the reef is an inevitable as life.

Voss said coral reefs grown where there is hard bottom in moderately shallow, unpolluted water that generally doesn't dip below a median temperature of 20 degrees centigrade.

The reef is the creation of little creatures called polyps, which in Florida are generally anywhere from one-sixteenth of an inch to a couple of inches in size.

The polyps, borne along on the sea, strike hard bottom and begin to deposit a limestone-like substance. They never stop leaving these deposits, which turn into the coral that grows beneath them. Each species of polyp creates a different pattern of this limestone, which accounts for the various kinds of reef -- deer horn, elk horn, brain coral, lettuce coral.

The reef includes - many different types of coral, so the landscape never gets boring.

Some fish, like triggerfish and parrotfish, feed directly on the polyps. Other fish use the reef as a hiding place. Predators come to feed on the fish that hide or the polyp-eaters. It's a crowded place.

The Florida reef is called a living reef because the polyps are alive and creating new coral all the time.

There are some observers who think intrusion of man and the changing water quality will kill off the reefs eventually.

Voss doesn't share that view. "I don't agree with these people like Jacques Cousteau who are running around making pictures of the reefs because they think that's the only way their grandchildren will know what a living reef was.

"The best evidence I can see is that there hasn't been a heck of a lot of change" in conditions since the reefs began forming after the last ice age, Voss said.

To make sure, Voss and others are about to embark on an extensive study of the Pennekamp reefs, mapping the numbers, kinds and distribution of animals and plants throughout the 30-mile length and six-mile breadth of the sanctuary. That will provide the base data to know it, if and when things do start to change dramatically.

If the reef is wonderful it is even more wonderful because it's named after a newspaperman. John Pennekamp, an editor of the Miami Herald, made it one of the paper's missions in the early '60s, to get the sanctuary established.