No. A half hour is good enough, says Cynthia Yoshida, M.D., a gastroenterologist at Martha Jefferson Hospital, in Charlottesville, Virginia, and the author of No More Digestive Problems (Random House, $14, available on www.barnesandnoble.com). Unless you’ve eaten a large, heavy meal, 30 minutes is plenty of time for some of the food to start emptying from your stomach, preventing your recreational swimming from giving you cramps.