The trouble with swimming, is that you have to pretty much arbitrarily designate areas you CAN'T swim to. A lot of the zones border oceans or large lakes. In theory, you should be able to keep swimming for hours, because these bodies of water are freaking big. Exploring areas like that would either be pointless, frustrating, or both - not to mention that if SE actually rendered these huge ocean areas, that'd be a whole lot of wasted resources.

So, SE would need to create boundaries to keep people from swimming too far. How would this be done? You could arbitrarily set invisible walls, but that is immersion-breaking. Boundaries on land can be excused by setting up steep cliffs or chasms (my character won't go further because it's too steep to climb!), but that's tougher to do in the water (my character can't go further because... well... the water is really, really hard for some reason!).