I would love to hear from the Physical Therapists and trainers out there on this one. I swim a lot for an old guy (3000 yards 3-4 mornings a week) with a bunch of other old guys and gals much faster than I. The pulling motion in free style, when done correctly, seems to imitate for the most part the follow through for a pitcher. Butterfly may also be good, but its a pretty hard stroke for most everyone, especially the part time rec swimmer. I can's see breast stroke helping much, though.
Rather than pulling on weights in the gym, you are pulling through or pushing against the water, so there would be no tendency to overdo it, or have competitions with your football playing friends in the weight room.
The repetitive stress injuries to swimmers come from the monsters who are doing 3000 yards before they brush their teeth every morning. I think that would clearly be overdoing it for a pitcher, but it just seems that a moderate workout a couple of days a week (lap swimming and maybe a few 25-50 yard sprints) can only help arm and shoulder strength.
No. 1 son (a pitcher) has started to swim a bit, but until they develop the Michael Phelps Ultimate Swimming Challenge for X-Box, I don't think he'll start serious swimming work.