So you don't have your own boat or an obliging friend to get you out on the Potomac. Just take matters into your own hands. Try lunch, dinner, dance or sightseeing tours. Rent a sailboard or a boat -- you can get a rowboat for as little as $5 a day.

Or perhaps you'd be more comfortable at Yachts America in the Fort Washington Marina, where $500 will buy a captain and crew for four hours aboard "The Holiday Mansion," a 40-foot cathedral-hulled cruiser that can handle up to 25 people while pulling skiers.

If you want to be the captain of your own powerboat, knock next door at Capital Sailboats, where Gordon Rutkai appears to be the only guy in town who rents motorboats. "Motorboats are a disaster to rent," he explains. "They are very hard to keep in working order." His prices start at $10 an hour.

One outfit, McLean Rentals (356-6262), will rent its only boat motor, a 7.5 horsepower, for $20 per day or $25 for the weekend.

Or maybe you want to buy your own motor for Potomac puttering. The smallest size recommended, a 7.5 horsepower, sells around town for $500 to around $1,000.

Upriver on the Maryland side opposite Harpers Ferry, River and Trail Outfitters will take the brave whitewater rafting and canoeing. And, for $95 per person, they'll give you a whole day to learn float fishing.

Following are some places that will help get you rollin' on the river. Remember that deposits are usually required, some boats are in short supply and boardsailers often must show training certificates. A phone call to the rental shop might avoid disappointment.

For general information on river conditions, two places to call are the Metropolitan Police Harbor branch at 727- 4582 or Interstate Commission on the Potomac River Basin in Rockville at 340-2661. ROW & PADDLE THE POTOMAC CAPITAL SAILBOATS -- Fort Washington Marina. Pedal boats: $6 an hour. Dories with 7 & 10 horsepower motors, $10 per hour; $20 for 3 hours; $33 per day. 292-2227. FLETCHER'S BOAT HOUSE -- 4940 Canal Road NW midway between Key and Chain bridges on Canal Road. Canoes, rowboats: $7.95 day, Monday to Friday; $9 day, weekends. Allows use of up to 10 horsepower gas or electric outboard motors on rowboats. 244-0461. HUDSON TRAIL OUTFITTERS LTD. -- 10560 Metropolitan Avenue, Kensington and 9683 Lee Highway, Fairfax. Canoes and kayaks begin at $18 day. Three-day minimum on weekends. 949-2515 in Maryland; 591-2950, Virginia. JACK'S BOATS -- 3500 K Street NW. 337-9642. Canoes and rowboats, $5 per hour, $12 a day. POTOMAC RIVER TOURS -- Beginner white-water trips, Viginia Great Falls National Park, seven-mile trip. Most weekends at 9. $26. 530- 8733 or 546-7200. RIVERBEND MARINA -- 8700 Potomac Hills Street, Great Falls, Virginia. Rowboats rented, $5 per day, $2.50 after 4. Hours 8 to 6:30, Monday to Friday; 6 to 6:30, weekends. Life jackets 50 cents. Can use up to 71/2 horsepower gas or electric motor on rowboats. 759-9018. RIVER & TRAIL OUTFITTERS -- On Valley Road off Route 340, Maryland side of Potomac opposite Harpers Ferry, West Virginia. Canoes, $25 per day, 8:30 to 6 every day. 301/834-9950. SPRINGRIVER -- 5606 Randolph Road, Rockville and 2757 Summerfield Road, Falls Church. Canoes: $18 a day; $28 per weekend. Kayaks: $18 a day; $25 per weekend. $5 for roof rack. Rafts: four-man, $50 weekend; six-man, weekend. 881-5696 in Maryland; 241-2818 in Virginia. THOMPSON BOAT CENTER -- Rock Creek Parkway and Virginia Avenue NW. Canoes: $3.50 an hour, $10 a day. Sunfish: $7 an hour. Sailboards: $8 an hour. 333-4861 or 333-9543. SAIL THE POTOMAC BELLE HAVEN MARINA -- One Belle Haven Road, Alexandria. 19-foot Flying Scots (holds one to six people) and Hobie Cats: Monday- Friday, two hours for $24; four hours, $42. Weekends, two hours for $28; four hours for $50. Sailboards, sunfish: Monday to Friday, $7.50 an hour; weekends, $9 and hour. Hours: 9 to dark, Monday to Friday; 9 to 6, weekends. Includes life jackets and rigging. Reservations required. 768-0018. CAPITAL SAILBOATS -- Fort Washington Marina. Rates begin at $8 per hour. Sloops, Sunfish and overnighters. 292-2227. POTOMAC BOARDSAILING -- 687 South Washington Street, Alexandria. Sailboards: $25 weekdays; $35 weekend days; $100 week. 836-7873. POTOMAC SKI AND SAIL -- 3610 University Boulevard, West Kensington. Sailboards, $40 a day. 949-6000. WASHINGTON SAILING MARINA -- Dangerfield Island, P.O. Box 1038, Mount Vernon Memorial Highway, Alexandria. 22308. Sloops and Sunfish, prices vary. 548-0001. THE WET ONES -- 163 Glyndon Street SE. Vienna. Sailboards, $50 a day. 938-1111. WINDSURFING UNLIMITED -- 7730 Old Georgetown Road, Bethesda. 951-0705. Sailboards, $10 an hour, $35 a day. 951-0705. CRUISE THE POTOMAC THE DANDY -- Potomac Party Cruises Inc., Zero Prince Street, Old Town, Alexandria. 683-6076 or 683-6090. Sunday lunch cruises leave at 12:30, $19.90. Evening dinner cruises leave at 7:30 nightly, $29.90. PRESIDENTAL YACHT CHARTERS -- 600 Water Street SW. 11 NBU 8-3, D.C. 20024. Charters from $120 an hour to $350 hour on 44-foot to 85-foot cruise ships. 362-8297. WASHINGTON BOAT LINES -- Sixth & Water streets SW. Nightly "City Light" cruise features narrated tour of waterfront monuments: $6.50 for adults, $4 for children. Friday night dance, 8 to 11, for $11. Daily Mount Vernon cruise, 11:30 to 4:30; $12 for adults, $10 for over-60s, $6 for children. Admission to estate included. Floating buffet, leaves 11:30 and 1, Monday-Friday; $8.75 for adults, $5 for children. Waterfront shuttle cruises: $4 roundtrip, $2.50 one-way. Leaves Pier 4 at 8:50 a.m. Stops at Lincoln Memorial hourly on half hour beginning at 9:30 and at Georgetown (bottom of Wisconsin Avenue) on the hour beginning at 10. 554-8000, 8011. YACHTS AMERICA -- 13602 King Charles Terrace, Fort Washington Marina, Fort Washington Road, Fort Washington, Maryland 20744. 40-foot "Holiday Mansion," a speed/house boat, holds 25 people. $500 for four hours, with crew. 292-1300.