ON A CINNAMON-COLORED beach, hundreds of swimsuit-clad children, teens and adults soak up the sun. A few wade into the breakers, where the waves, lapping around the knees of their elders, loom large to the three-foot-and-under set. In the near distance rises the surreal outline of the Bay Bridge, a gigantic spider's web tugging trucks and impatient city folk to the shore.

The place is Sandy Point State Park, one of four public beaches run by the Maryland Forest and Park Service along the Chesapeake Bay.

Nice outings for families, the Bay beaches offer the saltiness and wave action of the ocean beaches but with gentler breakers, generally shorter drives and, in some places, fewer people.

But, as with any natural environment, there are a couple of Ms. Nature's nastier elements present: sea nettles ("Not fun," says Sandy Point's assistant manager, Gerry Thompson) and mosquitoes ("Real healthy in the wilderness area," he reports).

Ouch. Why should people leave their comfortable, close-by, chlorine pools and come to Sandy Point? "Crabs," says Thompson.

"A lot of people rent boats here just to go out and see the Bridge up close," he says, "and look for crabs around the pilings."

Then there are birders who tramp through the fire trails in Sandy Point's Corcoran Tract -- a 130-acre wilderness area -- and stalk the shores, looking for ospreys. Or boat owners who find the park's 22 launching ramps cheaper and easier access to the Bay than the posher marinas around Annapolis.

The park, in fact, fills up most Sundays by 1, its 2,000 parking places overflowing. "They'll stay in line outside the gate, waiting for someone to leave, most Sundays," Thompson says.

With nearly 535,000 people using it last year, Sandy Point is by far the most popular of Maryland's public Bay beaches. "It's the most convenient to get to," says Barbara Rice, spokesman for the Maryland State Parks, "and with people driving by it on their way to the beach, a lot of people know about it."

The same can't be said of Heart Middle Island -- a site the U.S. Army Corps of Engineers used as landfill, north of the Bridge, that attracts over 140,000 people each year. "That's an amazing number," says Rice, "when you consider that you have to have a boat to reach it."

The 3,000-foot beach has roving lifeguards, but no facilities. "A lot of boaters like to go there, anchor and swim," she says.

Boaters visiting or camping at Janes Island State Park, near Crisfield on the Eastern Shore, also like to row out to its island for swimming. The chief draw there, Rice says, is "fishing -- most people come to camp and fish." Long pause. "But there are BIG mosquitoes."

People come, not for mosquitoes but for the water activities at Point Lookout, a park at the tip of St. Mary's County, where the Potomac reaches the Bay.

Swimming there is actually in the Potomac "because we like to keep it pretty well defined to one area," says park spokesman Susan Lynch.

The park also appeals to history buffs -- the area served as a prison for Confederate soldiers during the Civil War, a fact commemorated at the visitors center. And for $16 ($8 for children aged six to 12), you can take an all-day cruise from there to Smith's Island, settled in 1657 and populated, the cruise people say, by 750 descendants of the original settlers.

Those who choose to stay around the park will find plenty to do. Like Janes Island, it has camping sites -- but some of Point Lookout's have hookups. Roving lifeguards patrol the 900-foot beach, and bathers have access to a bathhouse and first aid station. And, not surprising for a place on two bodies of water, there is boat launching.

All the parks can be reached in one to three hours of easy driving -- or sailing -- from the Baltimore-Washington corridor, providing ready access to those who long to go saltwater daffy. BEACHES BY THE BAY

HEART MIDDLE ISLAND -- Managed by Gunpowder Falls State Park, Glen Arm. 301/592-2897. The island, in the Bay north of Bay Bridge and close to Back River, has roving lifeguards along its undeveloped beaches. No facilities. Accessible only by boat. Free.

JANES ISLAND STATE PARK -- Route 2, Crisfield. 301/968-1565. Most distant of the parks, it offers camping, picnicking, rowboat rentals and boat launching in addition to Bay swimming. Fees: $3 entrance ($4 for out- of-state cars); $3 boat launching ($4 out-of-state); $5 camping at primitive sites ($6 out-of-state). Take U.S. 50 east to U.S. 13; turn south. Take 13 to Rte. 413; turn south, and go to the end.

POINT LOOKOUT STATE PARK -- Scotland, at the tip of St. Mary's County. 301/872-5688. Visitors center, family and youth group camping (including 26 sites with hookups), nature and campfire programs, boat launching and rentals, a fresh-water lake for fishing, picnic facilities and lifeguards along the beach. Fees: $3 entrance ($4 for out-of-state); boat launching, $3; camping, $5 per night ($6 out-of-state), $7 for hookups and preferred sites. From the Beltway, take Rte. 5 south past Waldorf and St. Mary's to the very end.

SANDY POINT STATE PARK -- Annapolis. 301/757-1841. Facilities include a bathhouse, first aid station, boat rentals, a wilderness area with trails and picnic tables. Fees: $4 entrance ($5 out-of-state); boat rental, $7 per hour (plus $50 deposit); boat launching fee included in cost of entrance. Take U.S. 50 east past Annapolis; follow signs for park shortly before the Bay Bridge.