Hi,
beside usual swimming, you can try swimming with less breath - swim 50, 100, ... meters with breath on every 5th tempo. Actually, we use to train with a pyramid: 100m with breath on every 5th tempo, then 100m with 7 tempo, 100m with 9 tempo, then back: 7 tempo, and 5 tempo (3 tempo).
The distance and the tempo number depends on your fittness, of course. If you are fitt enough you can swim with 11 tempo breath. You can vary your rest time between the swims (or you don't keep any rest time).
Actually, the competetive swimmers also use this training method, I think.
Other good training method could be to swim 25m-s (50 m-s) on the surface (i.e. freestyle) with one breath, in series, where you set the rest time suitable for you. You can do it with fins as well.
Jee