More on swimming and stuff like that. Main advantage as far as I can tell from a non-kinesiology majour's perspective, is that it's basically a zero impact workout. If you can swim fairly well, check out masters swimming, it's competitive (sort of) which is a great motivator for some people and the level of skill of people there is wide to say the least, so don't feel bad if you've never raced before. If your campus has a pool, then it'll also be nice and convenient for between classes.
As far as knees go, just stay away from breaststroke kick (whip kick) cause that can be rough if you have a nagging injury, which I do as well.
This is all coming from an 11 year competitive swimming vet so there may be a touch of bias...
