* Old Town returns to the 18th century this weekend to celebrate spring and the sea. The Seaport Festival shoves off Friday at 6, when a royal couple is crowned to reign over the festival, after which they'll lead a parade down King Street. On Saturday at 10:30, Old Town waiters and waitresses will race down King Street balancing trays of filled glasses for a weekend in Williamsburg (second place finishers get $75). And Sunday, from noon to 4, Backyard Boats is offering free Hobie Cat sailboat rides on the Potomac. In addition, there'll be a couple of sport and sailboat shows on the waterfront, house and garden tours, and seafood specialties cooked up by local restaurants. WASH will be broadcasting live and giving away two vacation cruises in the Bahamas; register for the drawing Friday at 6, Saturday 11 to 3 and Sunday noon to 4 at the foot of King Street. Call 683-1135. Free. * Annapolis will also be steeped in merriment this weekend. To begin with, there's a parade that kicks off at noon on Saturday from Gate 3 of the Naval Academy, winds through the historic district and ends at the City Dock. Besides the parade, jugglers, magicians and aerobic acts will roam the downtown area and later, from 7 to 11, there'll be a dance at Annapolis City Marina at Fourth Street and Severn Avenue; admission is $2. On Sunday, a bull roast from 11 to 4:30 will feature live bands beginning at 12:30 at 222 Severn Avenue in Eastport. And don't forget about the continuous entertainment in the various parks from City Dock to Main Street until 6. * If you're both in Annapolis and in the market for a used boat, try the Annapolis Used Boat Show at the yacht basin on Compromise Street. More than 200 used boats will be on sale, from rowboats to offshore cruisers. Hours are 10 to 7 Friday and Saturday; 10 to 6 on Sunday. Admission is $3.75 for adults, $1.50 for kids under 12. There's free parking for the festival and boat show at the Naval Academy football stadium on Rowe Boulevard with buses going downtown for 25 cents (its free coming back). Annapolis city maps are available throughout the downtown area. Take U.S. 50 to Rowe Boulevard and follow signs for stadium parking. Call 301/268-7676.