This section seems very relevant to the swimming world:
How to Breathe in Abdominal Work
A question that comes up a lot for people who are learning to pull their abdominals in is, "If I have everything so pulled in, how do I breathe?" The answer is that we usually use a very small amount of our breathing capacity and tend to focus on the front body. In Pilates, we allow the breath to fully expand into the sides and down the back of the body. Doing so provides a lot of breathing room, and helps open and lengthen the back of the body as well.