You might say Harry Dodge is on a water diet. The 54-year-old Hechinger employee spends his lunch break saturated by the stuff.

A year and a half ago Dodge was concerned about middle-age spread when a friend mentioned something about a swim team here for older folks.

"The last time I'd done any competitive swimming was in high school," Dodge said, "but what did I have to lose?"

Thirty pounds, so far, and Dodge is still at it. He's doing laps in a pool as often as three times a day, seven days a week. His doctor is delighted that his blood pressure is down and Dodge says he never felt better.

He's one of 125 supposedly over-the-hill athletes who have found new life in Masters Swimming - a national program to encourage fitness and competition among adults 25 and over.

For Dodge and 47 other D.C. Masters swimmers, the culmination of a year's hard work came last weekend when they packed their trunks in their suit cases and flew to the national short course (25-yard pool) championships in Fort Lauderdale, Fla.

Dodge hoped to beat his best times in the 100, 200, 500 and 1,650-yard freestyle, racing against men in his age group - 50 to 54 - from all over the country.

The D.C. team's ages range from 25 to 83, and when the combined points are totaled up they'd like to repeat their showing of '75, when they swept both long, and short-course championships.

How do they prepare for this meet and the others they carpool to all season? They work their backfins off practicing by themselves or in organized sessions at four centers: Rockville Municipal Pool, Fun and Fitness Center in Arlington, American University and the Silver Spring YMCA.

John Flanagan likes to make other people work almost as hard as he does. A national champion in the 30-34 age group for the 100 and 200 butterfly and the 400 individual medley, he coaches practice sessions at Fun and Fitness.

"On a typical day I have swimmers going about 3,000 yards, depending on what meet is coming up and when it is. I also alter the total distance and the time intervals to suit the individual."

These sessions begin with warmup activities such as kicking, pulling, easy swimming or one-armed swimming. Workouts then proceed to timed sprints and longer swims. Time trials are held weekly.

Many of the swimmers also are on weight training programs to develop swimming muscles. Some even have equipment at home.

These workouts aren't only for the younger swimmers, either. Try to picture yourself keeping up with 70-year old Howard Smith. He averages about 1 1/2 miles of swimming daily plus a 30-minute session on weight equipment. In one year Smith has knocked almost 20 minutes off his time for a mile, down to 29 minutes. That's only about double the time for the best college swimmers.

At the other end of the range is Ellyn Morris.

"I was only 25, but already feeling old," she said. "I hadn't really exercised for a couple of years when a friend started raving about the Masters program. I decided it would be a good way to get back in shape and meet some people." Two years later she's dropped 30 pounds and formed strong friendships with people she calls "fellow fanatics."

Best of all, she is now swimming faster than she did in college. Good enough, in fact, to be the national champion in her age group for the 50-yard freestyle. She attributes a lot of her success to the coaching of people like American University's Joe Rodgers, who conducts Masters practice there on weekday evenings.

Best known of the team is Nellie Brown, pushing 84. After her performance in the Knoxville and Fort Lauderdale nationals in 1975, headlines proclaimed "Swimmer, 82, Captures Five AAU Medals."

All this despite the polio that left her with a shortened arm and leg and the three eye operations for which she wears special cataract goggles.

"I don't walk too good," she says "but just get me in the water. . ."

The 53 medals, ribbons and trophies she's won since joining the D.C. Masters two years ago are on display at the Alexandria YMCA, where she works out and teaches swimming to handicapped people and senior citizens. She is entered in six events in the nationals and won three in the short-course competition: 100 backstroke, 50 freestyle and 500 freestyle, but she admits the competition in her age group is not strong.

"Nobody swims against me, I swim against my own records."

Masters swimmers are in such good shape they don't mind getting older, especially if another birthday puts them in a different age group. Take Betty Brey, a member of the 1956 Olympic team and a veteran of several Pan American tours.

"I periodically race a former team mate from Purdue. She broke all my records in the 40-44 age group. Now that I've turned 45 I'm in another age group where I'll have a couple of years to relax before she's after-me again."

Brey, one of five original D.C. Masters, does find it difficult to find time for training.

"Younger and older people have more time. Women, especially in the middle-age bracket, seem to be more tied down with families," she said.

She tries to fit three or four training sessions into a week already crammed with her teaching job at Montgomery College, family responsibilities and other sports activities.

For many D.C. Masters, swimming is a family affair. Julie and Sandy Gideonse, 39 and 43, are cheered on by their three boys, who themselves are on swim teams. The Alleva family of Fairfax is so deep into swimming their decision to buy a house hinged on its proximity to the Starlit Aquatic Club.

Included in the team's $15 membership fee are dues to the local chapter of the AAU, insurance, a team newsletter, announcements of upcoming meets, and special charter rates to national competition. For details, call Dave McAfee, 532-775.