It's good fun exercise. The thing you gotta look out for is that you're not overtraining. With swimming the overhand motion with freestyle can't be good for our shoulders especially when it takes a beating everytime you throw a punch and more so when you land 1. Any repetive motion can wear down a joint over time. I don't see any problems with a boxer doing the the breaststroke.

As with any physical activity or sports that you may do on the side, make sure that it doesn't take away from your main endeavor.