I don't know why they do that can someone explain?
Most Helpful Guy
Never known any guys that do that (not that I've asked them what's under their bathing suits, mind you).
Some board shorts don't have an inner lining. If they don't, they plaster themselves to you when they get wet. Your bulge is prominent, the suit can chafe like all hell, and thin suits become see through if they're white. I suppose in that instance, it would be a good reason to wear boxers. Also, if they're particularly loose around the leg, it's an easy opening to showing your junk, so I suppose that might be part of it as well.
Most Helpful Girl
1. To keep things from rubbing together and chafing
2. In the event that the top suit falls off, everything is still covered
3. To prevent the butt crack from showing if the top suit slips (this is apparently different than reason #2)
Source: My boyfriend.