I'm not a competitive swimmer, but I read MMA Fighter's post, it seems logical. Though I think there are power gains, and soft twitch muscle improvements. But on a side note, yes swimming is a great cardiovascular exercise (mind you, cardiovascular, it has little to do with actual muscle strength). It is far less stressing on the joints, running isn't that great for you because of the pressure being hammered onto your knee/ankle joints. Plus it's a lot more multijoint movement then running, you'd be doing pretty much your full body with swimming. With running, well, you got your legs, your butt, and a bit of your core muscles, but thats about as far as it gets.