Here are some articles I found on swimming and baseball pitchers. The first two are against it, the third is for doing water exercises for pitchers. I think the general consensus is that actually swimming, ie doing freestyle, is not great for the shoulders. Even the article that talks about swimming being a good thing only talks about doing about 2 minutes of actually swimming, the rest of the workout is resistance exercises in the water.
Make your own judgements.
Against:
http://www.ericcressey.com/swimming-for-pitchers
http://www.baseballfit.com/baseball-training-0107.htm
For:
http://www.thecompletepitcher....itching_swimming.htm