The six beaming youngsters lined up on a Florida springboard are known across their home state as the "Six Swimming Sheas." Children of Mr. and Mrs. Eugene Shea of St. Petersburg, they normally gobble up 10 to 15 first places in a meet and have won a wallful of trophies and medals and enough ribbons to make a good-sized quilt. Nila, now 15, began racing at the age of 6 and two years ago won three events in the St. Petersburg women's championships. Last year she set a new U.S. junior record in the 200-meter freestyle and Diane, 13, holds Junior Olympic marks for her age group in the 50-yard backstroke and 50-yard freestyle. Eleven-year-old Dennis posted the second best time in the country in two events in 1954 while Danny, 9, shows promise of soon catching up to his older brother and sisters. Dawn, 5, and Dana, 3, are the latest Sheas to be trained by their mother, a former ocean swimmer and lifeguard.