"Sailing in this area is probably at its best this time of year," says Glenn Hall, 14-year veteran of the seas and boat repairman at the Washington Sailing Marina. "People are tired of being in their houses all winter. In the spring you have warm weather and good winds up through July. Then the wind gets light and variable and the humidity goes way up until September."

Area marinas are getting ready for a busy rental season as spring sailors and sun- worshippers pack away their winter wear and bring out shorts and swimsuits. But expert sailors offer some words of warning to any first-time spring sailor: Bring along some warm and waterproof clothing and don't forget to pack a hearty sense of adventure -- and be prepared for cold water.

Hall advises sailors to dress for cooler weather than anticipated -- it's usually colder on the water. "Flannel shirts, windbreakers and sweatshirts are a good idea," he says.

Gordon Rutkai, sailboat specialist at the Fort Washington Marina, tells sailors to bring along some "foul-weather gear or other waterproofs. Remember, you're going to get that occasional splash of water this time of year."

Soft-soled shoes, a hat and sunglasses should also be included on a sailor's checklist. If the wind picks up and the temperature drops while you're out, a thermos of warm tea or coffee will come in handy, too.

For inexperienced spring sailors, Rutkai recommends renting slightly larger boats than you would in the summer months. "The water is cold and you want to be out in a relatively stable, dry boat this time of year," he says.

George Stevens, owner of Belle Haven Marina, a mile south of Old Town Alexandria, tells rookie spring sailors to go out on days when there are comfortable, light breezes. He warns against bringing the whole family out on blustery days and adds a stern reminder about putting on lifejackets -- especially on small children.

Next to his marina overlooking the Potomac and the boats, there are about 30 picnic tables with grills. "This is a beautiful place to sail. There are no factories or highrises, just trees on either side," he says.

Most boat rentals open early in the morning and close at dusk. Reservations are mandatory on the weekend. At the Thompson Boat Center, Virginia Avenue and Rock Creek Parkway NW, no reservations are taken any time. The best thing to do is to get there early and wait, says Thompson employee Betsy Broughton. They'll be open from dawn to dusk starting April 1. SAILS FOR RENT ANNAPOLIS SAILING SCHOOL -- Rentals start March 26. Boat available: Rainbow (24 feet), $90 per day or $20 per hour. Advisable to reserve at least one week in advance. To get there, take the John Hanson Highway (U.S. 50) east to Rowe Boulevard (route 70), continue to Church Circle and turn right on Duke of Gloucester Street, cross the bridge into Eastport, and after three blocks the school is on the left. 261-1947 (D.C.), 301/267-7205 or toll free 800/ 638-9192. CAPITAL SAILBOAT AGENCY: FORT WASHINGTON MARINA -- Rentals start April 1. Boats available: Sunfish, $8 per hour or $22 for three hours, $36 for day; Apollo 16 (16 feet), $10 per hour, $26 for three hours, $48 for day; Newport 17, same rates as Apollo 16; DS 16, same rates as Apollo 16; Explorer 21 (21 feet), $12 per hour, $35 for 3 hours, $60 for day; American (24 feet), $15 per hour, $40 for three hours, $75 for day. Reservations not necessary, but call the day before to be sure they aren't booked up. Located next to Fort Washington National Park in Fort Washington, Maryland. 292-1300. THOMPSON BOAT CENTER -- Rentals start April 1. Boats available: Phantom (12 feet), $7 per hour; Windsurfer, $8 per hour; canoes, $4 per hour, $10 for day. No reservations. The center is on Virginia Avenue and Rock Creek Parkway NW. 333-4861. WASHINGTON SAILING MARINA -- Rentals start April 1. Boats available: Sunfish (13 feet), $7.50 per hour; Designers Choice (15 feet), $10 per hour. Open 10 until dusk, but hours will expand as days get longer. Located off the G.W. Parkway, next to National Airport. 554-0222. BELLE HAVEN MARINA (MARINERS SAILING SCHOOL) -- Rentals start April 9. Boats available: Flying Scots (19 feet), weekday: $24 for two hours, weekend: $28 for two hours, half-day rate: weekday: $42 for four hours, weekend: $50 for four hours; Hobie 16 (16-foot Catamaran), same rates as Flying Scots; Sunfish (14 feet), weekday: $15 for two hours, weekend: $18 for two hours, half-day rate: weekday: $25 for four hours, weekend: $30 for four hours; Windsurfer, same rates as Sunfish. Located off the G.W. Parkway south of Alexandria on U.S. 1. 768- 0018. GANGPLANK MARINA -- Rentals begin May 1. Membership cost of this sailing association is $300 for the season; $170 for half-season. If you belong to the marina, sailing is free. Boat available: Albacore (15 feet). Rental season runs through October 31. Informal races are held Sunday for members. Non- member, experienced sailors can charter larger sailboats for either a half-day or entire day. Boats available: Bluenose (22 feet), $50 for half-day, $80 for day; Cape Dory (22 feet), same rates as Bluenose. The marina is at 600 Water Street SW, opposite Arena Stage off Maine Avenue SW. Call Capt. Giles Kelly at 244-5633..