Washington attorney Diddo Ruth Clark joins three other women and 11 men in the first Manhattan Island Swimming Marathon today.

Clark, 32, will be trying to break Diana Nyad's record of 7 hours 57 minutes 31 seconds set in 1975 for the 27-mile course around Manhattan Island.

"The conditions are terrific," Clark said Sunday, following an afternoon practice swim in the East River. "The water is a lot warmer than I expected (70 degrees). In fact it is just the temperature I would choose. The water was cleaner too. I could see my hands with my goggles on."

Drury Gallagher, president of the Manhattan Island Swimming Association and the men's record holder at 7:57.44, said yesterday, "The toughest part of the race is the first hour in which you have the roughest currents and tides going through Hell's Gate."

Hell's Gate is at the confluence of the East River and the Harlem River where the currents meet to form circular whirlpools that push the swimmers around. The race begins at the 89th Street Pier and moves counterclockwise around the island via the East River, through Hell's Gate, up the Harlem, down the Hudson and back up the East to finish at the starting point.

The first successful swim was completed by Robert W. Dowling, then 18, on Sept. 5, 1915, in 13 hours 45 minutes. Fewer than two dozen swimmers have completed the swim.

Clark, an employe at the Department of Labor where she reviews regulations, said, "I've become a swimming junkie and I work to support my habit.

"After I swim I feel so good I can hardly stand it when I go to work. As I understand it, our body produces endorphins which gives us a high. It is a physiological high."

Clark started open-water swimming as a child during Christmas vacations to Mazatlan, Mexico. The warm ocean water of the Pacific plus the lure of a couple of offshore islands sitting just two miles away was too much to resist, even for a person whose greatest fear is dark water.

Clark practices daily with the D.C. Masters swimming team at Northern Virginia Fun and Fitness under the coaching of John Flanagan, swimming about three miles every morning before going to work.

"One of the reasons I like the longer swims is that there is more information," Clark said. "I get splits, different feelings, thoughts and there is a lot more strategy."

Clark spent a week in July swimming in the cold water of the San Francisco Bay. She culminated the stay with a swim to Alcatraz.

"I was so comfortable in the water, so at home I just had to watch the scenery. I saw the Golden Gate Bridge, the ships, the city--I could see everything perfectly clear. It was one of the most beautiful experiences of my life."