The reason swimming makes you need to pee. Hear about it at Urban Legends and Stranger truths. Swimming is a terrific low impact exercise, which is why plenty of people make it part of their regular fitness regime. But if you spend enough time in the water, you might notice that your bladder seems more active than when you're on dry land. One study found that a 110,000 gallon pool contained roughly eight gallons of urine. Does being immersed in water suddenly increase your urge to go?