With the ongoing cleanup of the Potomac River -- and the more pleasant riverside environment that cleanup has created -- waterfront activities are once again a summertime option. Some suggestions for getting out on the water amid the cooling summer breezes: Ships, Boats and Barges

Lightship Chesapeake: After 40 years of service as a floating lighthouse, the Chesapeake was retired from the Coast Guard in 1971 and, after refurbishment, was moved by the National Park Service to her permanent mooring site off East Potomac Park in the Washington Channel. There are free tours daily from 9 to 5 (soft-soled shoes are recommended for your visit), as well as special educational programs each month. For example, this month, on May 17 at 10 a.m., Lightship Chesapeake is holding a water safety program related to swimming and boating, and on May 26 at 1 p.m., a park ranger will show different types of pollution be testing river water. 426-6896.

Paddle-Boating in the Tidal Basin: Across the Tidal Basin from the Jefferson Memorial, on Ohio Drive, you can rent a craft and paddle your way around the waterway. Boats, at $2.45 an hour, are availabl every day from 11 a.m. to 7 p.m. 488-9730.

Tall Ships: Two of the famed square-rigged, three-masted sail training ships featured in the Bicentennial celebration, are the centerpiece of a variety of events along the Washington Channel called HarborFest, sponsored by the Washington Waterfront Association. The S.S. Christian Radich from Norway, which arrived here last Thursday, will be at Pier 4, 6th and Water Streets SW, through Tuesday, will be at Pier 4, 6th and Water Streets SW, through Tuesday, May 6. It will be open to the public today from 11 a.m. until 4 p.m. On Friday, the Pier 4 berth will be taken by the S.S. Danmark from Denmark, which will be open for public visits on Saturday, May 10 from 1 p.m. until 4 p.m. and Sunday, May 11 from 10 a.m. until 4 p.m.

The Diplomat, a regular occupant of Pier 4, 6th and Water Streets SW, chugs down to Mount Vernon and back every day. Departures are at 9:30 a.m. for four-hour cruise, and at 2 p.m., when the cruise lasts four and a half hours, with layovers at Mount Vernon. Round-trip fare for adults is $6 ($3.50 one-way), $5 for senior citizens ($3 one-way), $3 for children ($2 one-way).

The Spirit of '76, a narrated sightseeing tour between the Lincoln Memorial and Georgetown, departs daily from the Lincoln Memorial dock, south of the Memorial on Ohio Drive, between 11 a.m. and 3 p.m. every hour on the hour. From the Georgetown dock it departs on the half hour between 10:30 and 3:30. Fares are $3.25 for adults, $2.75 for senior citizens and students, $1.25 for children. One-way fares are $2, children 50 cents. Round-trip fares permit you to disembark at either dock, reboarding for return later the same day. Tuesday through Sunday evening champagne cruises leave the Georgetown dock at 6:30, 7:30 and 8:30, from the Lincoln Memorial at 7 and 8. Both the Diplomat and Spirit of '76 are available for private charter. For information or reservations, call Wilson Boat Lines, 554-8000.

You can guide into the 19th century on the "Canal Clipper," the C&O Canal's mule-drawn barge that leaves from Great Falls Tavern, Potomac, Friday through Sunday until June 1, Wednesday through Sunday from June 4 through Sept. 7. The barge is open to the public for one-and-a-half hour rides at 11 a.m., 1:30 and 3:30 p.m. Reservations-only evening charters are from 7 until 10. For information and reservations, call (301) 299-2026. The Friday-Sunday schedule resumes Sept. 12 through Oct. 26. Adults, $2.50; senior citizens and children 12 and younger, $1.25. Group evening charters, $400. Call 229-2026. Learn to Sail

The Fairfax County Department of Recreation and Community Services will offer a boat load of sailing courses for children and adults at bile-bottom rates. For adults instruction is available for eight 2-hour sessions on 19-foot Flying Scots starting June 18 with theory classes and winding up on your choice of weekends -- June 21-22 or June 28-29 -- in the boats. The cost is $64 per person. Adults (16 years and older) may also take instruction on 14-foot Sunfish sailboats in five 2 1/2-hour sessions Mondays through Fridays beginning the week of May 26 and through the week of July 28. The cost is $47 per person. Youths have three choices of programs. Courses taught in Flying Scots for 10 to 15 year olds run from June 23 to July 3 and July 17-18. The cost for the six 3-hour sessions is $50. Instruction in Sunfish sailboats for 11 to 15 year olds will be given June 23 to 27 and July 14 to 18 for $47. A youth sailing day camp of five 6 1/2-hour sessions will be offered July 7 to 11, July 21 to 25 and July 28 to Aug. 1. The cost for 11 to 15 year olds taking the camp course is $70. Transportation for most classes from points in Fairfax County is also available at nominal fees. Call Pat Archer at 691-2672 for more information.

The Mariner Sailing School, which will operate some of the Fairfax County sailing courses listed above, also offers adult and youth sailing instruction at the marina in Belle Haven Park on the super-stable Flying Scot and the sporty Sunfish beach boat. Adult learn-to-sail courses include 10 hours of instruction in the boats on the water in four 2 1/2-hour sessions with a maximum of three students and one instructor per boat. The course is $90 for one person, $180 for two and $260 for three. Courses are taught every day of the week except Friday.Youth sailing instruction includes 18 hours of instruction for $50 and is taught on the Sunfish. Flying Scots are available for rental at rates from $16 for 2 hours on weekdays to $35 for 4 hours on weekends. Sunfish rentals are $9 for two hours on weekdays and $12 for 2 hours on weekends. The school plans to offer Friday evening racing in Flying Scots on a rental basis beginning later this month.

You can learn to sail at the Washington Sailing Marina and then keep up on your skills at lunch. Located only a few minutes south of National Airport on the George Washington Memorial Parkway, the marina plans to offer reservations for rental boats during lunch hours this summer. Instruction for ages 16 and up will be offered in Sunfish sailboats Monday through Friday evenings from 6 to 8:30 starting the week of May 26. The cost of the course is $50. Youth classes will be held days June 23-27 and July 14-18 for $50 per child. Youth day camp sailing instruction will be offered July 7-11, July 21-25 and July 28-Aug. 1. The cost for the day camp course is $80.Twelve-foot Widgeons and 14-foot Phantom sailboats can be rented for lagoon sailing and a new day sailer, the 15-foot Deigner's Choice, can be rented for river sailing. Private sailing lessons are also available at $20 an hour. The marina is open every day from 10 a.m. until dark weekdays and 9 a.m. until dark weekends. If you bring your own boat, launching facilities are available for boats under 2,000 lbs. Call 548-0001 for more information.

The 15-foot Albacore day sailer has one of the largest racing followings in the Washington area and you can learn to sail it at Gangplank Marina, 600 Water St. SW. The sailing course includes 16 hours of instruction in Albacores for $125 and is held Wednesday evenings and Saturday mornings. Graduates may join an asociation at the marina for a fee to get use of the Albacores. Racing classes will be held on Sundays. Private lessons are $18 an hour. A Venture 22 sailboat with overnight accommodations is available for $150 a weekend. Call Giles Kelly at 244-5633.

Buzzard Point Marina at the foot of 1st St. SW has five boats available for rental from a 14-foot Sunfish to a 20-foot Ensenada at rates from $5 to $15 an hour. A 10-hour sailing instruction course costs $120 and individual instruction is offered at $16 an hour. The marina also sponsors a sailing assocciation for members with unlimited use of a special fleet of boats for $400 a season -- May 15 to Oct. 15. Call 488-8400. I Canoe, Canoe Canoe?

For non-sailers, Fletcher's Boat Hose at Reservoir and Canal Roads NW, offers multiple choices. Aluminum canoes, three-speed bicycles and flat-bottom fishing boats are available for rent by the day and hour. Canoes are $4 an hour and up to $7 a day on weekends. Bicycles rent for $5 a day or $1.50 an hour and fishing boats are $6 a day on weekends. Bait and tackle are also available. Fletcher's is open from 7:30 a.m. until dark on weekdays and from 6 a.m. until dark on weekends. Call 244-0461 for more information.

Two other options for novices interested in a day on the water are Thompson's Boat Center, located opposite the Watergate at Rock Creek Parkway and Virginia Avenues, and Jack's Boats, located under the freeway in Georgetown at 3500 K Street. Thompson's is open from 10 a.m. to 5 p.m. daily and rents canoes and rowboats for $3 an hour, $9 a day. You must be 21 to rent. Jack's rents rowboats and canoes for $4 a hour, $10 a day and is open from 10 a.m. to 8 p.m. Both boat houses also rent bicycles. For more information call Thompson's Boat Center at 333-9711 and Jack's Boats at 337-9642. Any Old Thing That Floats

If you're planning to build a raft this year for the annual Rambling Raft Race, you have until Aug. 23 to complete your craft. Rafts will be launched at 9 a.m., but all participants should arrive at West Potomac Park two or three hours earlier to register. The race lasts until 6 p.m. and prizes and trophies will be awarded. For all the rules and regulations, keep your ears posted to Wpgc (1550 AM, 95.5 FM) or call Sherry Berger at 779-2100. More information will be available a month before the race. Rock Creek Park

Rock Creek Park, one of Washington's -- and the world's -- most beautiful parks, will turn 90 this September. During a recessionary summer that may make exploring farther from home too expensive to consider, you might consider the outdoor and sports opportunities that our nonagenarian, and free, natural wonder has to offer.

Geologically, the park's a lot older than 90, though. The rocks and boulders along parts of Rock Creek that mark the boundary, known as the Fall Line, between the hilly regions to the west and north called the Piedmont Plateau, and the Coastal Plain to the east and south, are all remnants of an ancient mountain range. Those remains can be spotted easily along a portion of Rock Creek between Sherrill Drive and Tilden Street. The Rock Creek Nature Center, 5200 Glover Road NW (see map), has exhibits and programs that explain these natural features, as well as materials for self-guiding nature hikes and other interpretive aides. Conducted nature walks and hikes start at the Center and other park locations. (There are 15 miles of trails with footbridges in addition to the bridle trails which hikers may also use.) The center is open Tuesday through Sunday, 9:30 a.m. to 5 p.m.

If you'd rather ride around the park, Rock Creek Horse Center, near the Nature Center, offers rental horses and instructions. Rides on Tuesday through Friday begin at 1:30, 3, 4:15 and 5:30 p.m. Saturdays, Sundays and holidays there is an additional ride beginning at 11:30 a.m. The fee is $5 weekdays, $6 weekends and holidays. No reservations are taken unless you pay in advance.

For picnicking, there are about 70 groves in the park complete with tables and benches and, frequently, a fireplace. Some of the groves may be reserved ahead of time through the D.C. Department of Recreation, 673-7647.

The park has facilities for football, baseball, softball, volleyball, horseshoes, archery and tennis. The tennis courts may be reserved through the Government Services Inc., at 723-2669. There's also an 18-hole golf course along Beach Drive above Military Road, complete with pro shop, locker rooms, lounge and refreshment bar. The greens fee is $2 for 9 holes, $3 for 18 on weekdays; $2.50 for 9, $3.75 for 18 on Saturdays, Sundays and holidays. The entrance to the golf course is at 16th and Rittenhouse Streets, NW.

You can also take to the woods for musical entertainment. Carter Barron Amphitheater offers summer evening programs outdoors. For details on this summer's cultural offerings, see our calendar beginning on page 20. Or call 829-3200 for more information and tickets.

Should you need help, the Park Police have an office on Beach Drive, south of Military Road. Call 426-6680.

One long-time favorite for Rock Creek outings is Peirce Mill, built about 1820 by Isaac Peirce and his son Abner, who ran this gristmill for some 80 years. Restored in 1936, the mill is open free to the public Wednesday through Sunday, 9 a.m. to 5 p.m.

The park is also famous for a number of Civil War and War of 1812 landmarks. During the Civil War, Military Road was first used for transporting material to build and later supply Forts Reno, DeRuss (see map) and Stevens, three in the chain erected in the early 1860s to protect the approach to Washington.

Milkhouse Ford (see map), served as the crossing from Chevy Chase to Bladensburg for the American forces marching to Battle of Bladensburg in 1814. And it was the retreat route for those forces as the British marched toward Washington to burn the White House.