I have two HS ballplayers that have been swimming competitively since they were young, and full-time on competitive club teams for the last 5 years. The only time they are not swimming regularly is during the HS baseball season. The younger swims AAA times, so he is obviously working quite hard in the pool. Both pitch quite a bit also - about 100 innings a year between HS, travel, and fall. Neither has sustained any arm injury other than general soreness. So, anecdotally, I can say that swimming has complemented their pitching quite well and has seemingly helped their durability. It goes without question that it has helped their fitness, although swimming doesn't do a whole lot for outright speed. It does work the core, and as noted in another post, stabilize the shoulder with strength in all directions. I am very much in favor of swimming for pitchers.