They say that swimming is one of the best excersises for you. It is the easiest on your joints/body, but the most effective in moving muscles that might otherwise not get used much. They use water for disabled people on a regular basis because the movement of the water again moves those muscles that are not normally used by them. Go for the swimming for exercise for now. Who knows, you might really enjoy it and start thinking about tri's (because we know that no cycling is not an option)