Swimming is the best workout for you. I'm a boxer, I do martial arts, and I played many high school sports, and played baseball at the Uni level. A boxing match is the hardest thing to endure, but I will say that to get in shape, swimming is the best for you.
It will drastically improve your cardio, and it works muscles in your body that lifting alone won't.
Think of it this way. You body needs calories to feed your muscles. The more muscles you work, the more calories your body will burn. Swimming will burn the most at one time.
A matching healthy diet will be the biggest help, though.
Do it, do it, do it.