Swimming isn't easy to animate in a video game. First of all there's the complexity of underwater effects. Then there's the freedom of movement in three dimensions, which is neat for the player, but a real pain to achieve in third-person games in terms of achieving an intuitive interface and a fluid camera. Then there's the problem of water meeting land -- it's easy enough to have your character jump or fall into water, but to get them out again you'd need ladders, shores, so forth, so you're going to have to do your art direction with that in mind. And to be even slightly realistic you'll need to think about the character's oxygen supplies. Basically, having your character be able to swim means adding an entirely new form of gameplay that's usually boring, annoying and nowhere near worth