MIMI CHOMEAU was there because she has lived within half a mile of the Potomac all her life. She knows it can be dangerous but wants to canoe on it. Richard and Joan Porter were on hand because they hope to take their young children out in their new canoe. "We don't want to take chances with them," Joan Porter said. "We want to learn all we can so we can enjoy the river."

Also in attendance were a troop of Boy Scouts, several couples and a few parent-child pairs, all come to Swain's Lock on the C&O Canal for a free canoe lesson courtesy of the Canoe Cruisers Association. The CCA, Washington's largest canoe club, provides volunteer instructors, and the Swain family, for whom the lock is named, provides canoes, paddles and life jackets.

"In one evening, we hope to get you to paddle up and down the canal in a straight line without screaming at each other," said John Seabury Thomson, an infinite gentleman who has been teaching the free classes almost from the time they began in 1972 at the request of the National Park Service. They're held Tuesday evenings at Fletcher's Boat House, Thursday evenings at Swain's.

"The first consideration in canoeing, always, is safety," Thomson continued with the students, stressing the importance of wearing life jackets. Then, in a few carefully chosen paragraphs, he warned them about the dangers of the river, a hundred yards or so behind him, beyond the trees.

Being able to paddle straight in the canal is not adequate for paddling on the river, he told them, but it is an introduction to the sport, a chance to test themselves.

Thomson demonstrated the basic paddling strokes and how to get in and out of a canoe. "Those of us who paddle whitewater kneel in the canoe," he said, "but you may want to sit. If you do, though, the canoe will feel more tippy."

As the nervous students began to board canoes for the waterborne part of the lesson, the other instructors waited in boats, positioned along the canal to guide, suggest, teach and cajole.

Most of the class paddled tandem but there were a few soloists. Most had absolutely no control over the canoe.

They zigzagged from one edge of the canal to the other; switching paddles from side to side; crashing into the bank, or each other, or the instructors. Bumper boats on the C&O.

Then, one by one, the instructors picked a student boat and began to paddle alongside offering suggestions: "Put your hand on the handle rather than the shaft of the paddle"; "keep the paddle shaft straight up and down when you put the blade in the water"; "try a shorter stroke"; "turn the thumb of your top hand down on the steering stroke."

Some students proved to be quick studies; by the end of the lesson the instructors were showing them the more complex river strokes.

And most of the her students at least were paddling in a straight line. No more bumper boats -- until next week. CLASS ACTS

The free lessons on the canal are 6:30 to 8 p.m. Tuesday nights at Fletcher's Boat House on Canal Road across from the intersection with Reservoir Road; and on Thursday nights at Swain's Lock, which is at the end of Swain's Lock Road, off River Road, about four miles north of Potomac. The lessons run through mid-September.

No advance registration is required, and there is no limit on how many can attend. Repeat visits are welcomed.

The Canoe Cruisers Association also offers beginner and whitewater classes on weekends. A $10 contribution is requested. However, registration for those classes is already closed. For more information, call 532-7551.

GETTING THERE -- To get to Fletcher's Boat House, travel north from M Street on Canal Road (one way at that hour on weekdays). Take the first left, which is just beside the white stone lockhouse across from Reservoir Road.

To get to Swain's Lock, take the Beltway to the River Road exit. Take River Road toward Potomac; 2.2 miles after the intersection of Falls and River roads, turn left onto Swain's Lock Road (a gravel road) and follow it to the end.