Humans can't breathe underwater. We can hold our breath for a short time, some of us longer than others. Longer than two minutes usually results in unconsciousness (and you'll automatically start to breathe again, which is very bad if your head is underwater). However, this seems not to be the case with some Video Game characters, who can swim underwater for infinite amounts of time and show absolutely no signs of discomfort. This incredible feat is never ever explained, or even mentioned as something out of normal in these games.