They don't bathe so when they get in the water their bodies absorb so much water that it weighs them down and they go under. Regular bathing prevents this from happening by allowing the body to slowly absorb and maintain a certain amount of water. It's like drinking beer. If you drink a lot over time you need to drink more to get drunk and feel the effects. But if you don't drink a lot then one beer can make you drunk. Totally true. They are getting drunk off of one swim.