I don’t agree with a point that diet is the best way to go, the best way is to combine both. Increased activity increases glucose metabolism, decreased weight increased glucose metabolism.
Diet is very important but if you can I think its much better to work on both so swimming is a great idea.
The trick with swimming is doing it at a pace that gets your heart rate up a little bit - whatever that pace may be. Doing 30 lengths of chin up breaststroke does not do a great deal apart from helps stretch out your muscles a little bit unless the pace your doing it at gets your heart rate up a bit.