Swimming will put on good muscle, but not definition. Unless you are in very hot water, your body will put on a layer of fat to combat the cold water.
The only way to get good definition in a short period of time is high repititions (running, lower weights, cycling) over high weights. Definition is the ability to "see" your muscles, so you need to lose fat. You can get larger from higher, and different, weights. If you combine the two, you have the best of both worlds.
More weight = size
More repetitions = definition
P.S. I used to tread water for an hour at a time as well as swimming but I never lost the body fat, got in great shape though. (I was a rescue diver, hence the training)