The view from the harbor of Old Town Alexandria is of a wide stretch of river where speedboats race to the Wilson Bridge and back again, cabin cruisers forge their way toward Chesapeake Bay, sailboats meander and drift on a calm day and sails of windsurfers dance in the distance. Seen from the park at THE FOOT OF PRINCE STREET, a small boat is growing on the horizon. The sail tacks in a promising direction; soon, its number can be read by the people sitting on benches among faded chrysanthemums. The sailor ties up, his wife disembarks, he pushes off again. As the sail catches the wind and pulls the boat to sea, the man props up his feet, stretches and eases back, hands behind his head. Waiting on land for something to happen has been rewarded with a closer look at the boaters. I of the rivet's like solving a mystery. To landlubbers who know not a mainsail from a jib and plan to keep it that way, there's nothing so beautiful as the Potomac River when seen from its banks, nothing so amusing as watching someone else playing on it. Shortly, a speedboat pulls in and docks at Prince Street. The occupants, three men and a woman, are making sandwiches on the forward deck, with a bottle of mustard perched above the dashboard. The captain tries to interest one of the picnickers in the park in a sandwich. One of the men leaps out, dispatched to Deli on the Strand, a block away, and returns quickly with two six-packs of beer. It is clearly easier to find a parking space in Old Town if you command a boat. A few blocks north, FOUNDERS PARK is designed for the landlubber. At Robinson Terminal, on the north end of a rocky strip of beach, the Corner Brook, out of Monrovia, was taking on cargo the other day, sharing the harbor with windsurfers. The park is a perfect spot for picnicking, fishing, biking, playing volleyball or just watching the waves generated by motorboats way out there. It runs beside North Union Street, starting at Queen and ending at Oronoco. Just below National Airport you can wander WASHINGTON SAILING MARINA's streets of ships -- sidewalks lined with small craft -- checking out where the boats come from or chatting with the owners about how they maintain the mizzenmast and so forth. Under the locust trees, picnic tables face the river and the airport; the engine noise is something you can get used to. A little restaurant sells Marinaburgers and Neptune Twists. Heading toward the District along the GW Parkway, crossing over Memorial Bridge to the D.C. side, follow the road along the river to WEST POTOMAC PARK. You'll be looking at the underbellies of jets as they land at National, stirring the willows at waterside. On the river, an occasional cabin cruiser takes a bearing for downtown instead instead of heading south for the Chesapeake. Next stop for land vehicles is EAST POTOMAC PARK. Here the river widens. Gulls call. A sunbather in a bright pink bathing suit fusses with her blond hair. Five children play tag on the bank, oblivious to the choppy gray water rustling beside them and the airport roaring across the way. At the tip of HAINS POINT, sailboats play; on land, Lilliputians climb on the hands of a half-buried sculpture of a giant. Coming around the point, next to a miniature golf course, the old presidential yacht Sequoia is docked in WASHINGTON CHANNEL: regretfull are practicing. Wilkins is the kind of person who gives a landlubber vicarious pleasure: He's visited the Three Sisters. He says picnics on the small islands above Key Bridge are over- rated: "There aren't many places to sit. It's real rocky," he said. "There are weeds sticking out of the cracks." The crew teams for GW, Georgetown and Trinity College, which row out of THOMPSON BOAT CENTER, may go as far north as Fletcher's boathouse, and as far south as Wilson Bridge, depending on the weather. In the traditional, collegiate, eight-person shell, teams can be seen practicing in earnest when school starts -- in the very early morning and the mid-afternoon, but not usually on Sundays. You can watch them from the grassy area in front of the boat center near the Kennedy Center or get a bird's-eye view from the walkway on KEY BRIDGE. The blade design identifies the school: Georgetown has gray with a blue tip (or blue with a gray tip, on the reverse side); GW men have navy blades with five yellow vertical stripes; GW women have yellow with three vertical blue stripes; Trinity Colrivelege has purple blades with three gold stripes; and Potomac Boat Club, red with a white star mid- blade. Crew practice can also be observed from THEODORE ROOSEVELT ISLAND. Below Key Bridge on the Virginia side there's a footbridge over to the island, but some people picnic on the bank facing the Georgetown waterfront. Canoes float in from Fletcher's or Thompson to make port at the island. The Spirit of '76 tourboat, which circles from Georgetown to 14th Street Bridge to Lincoln Memorial and back, sails past here. At the FIRST OVERLOOK north on the GW Parkway after Key Bridge, hop up on the rock wall and peer down at Fletcher's boathouse. From this height, watch the patterns the canoes trace in the placid water as they give each other wide berth. See how wide the arcs are, how far the ripples flow from a single canoe. The canoes and the boathouse appear to be just about H-O scale. There's rough water north of here and, with it, the most exciting of all water sports to watch on the Potomac -- whitewater canoeing or kayaking. Races are usually held in the spring in Mather Gorge, visible from the BILLY GOAT TRAIL on the Maryland side and from the RIVER TRAIL, down from the visitors' center in Great Falls Park, Virginia. But as long as it isn't the dead of winter, kayakers or canoeists in covered canoes (C-1s and C-2s) hotdog in the riffles and the rapids along this stretch. The Billy Goat Trail is reachable by walking north on the towpath from Old Angler's Inn or south from Great Falls Tavern. The drop from the rough, steep trail is 50 feet into the gorge: "It's dangerous," says Ivan Jorgensen, a park technician in Great Falls. "You have to watch your balance, you have to have good shoes. It's a rough, rocky trail. But the view is worth it." It's not recommended for children. If the kids want to see some kayakers up close, take them to the FEEDER CANAL near Lock No. 6 of the C&O. Walk upstream on the towpath and look toward the river. Often kayakers practice here, weaving among gates in the fast water as they would in a race. Without too much effort, you can hike upriver from Fletcher's or from Chain Bridge on the Maryland side to look upstream at the LITTLE FALLS section of the river, where, occasionally, expert kayakers or canoeists fight the waves. "I really stress that they are experts," says park ranger Katie Herlihy, "not just good at the sport. We do not recommend it for anybody." Makes you glad you're on dry land. After all, somebody has to be here to receive messages in bottles.