Interesting thread! I've posted before on swimming and how it helped my son. But he swam on competitive swim teams from the time he was 4 to about 12. He was never very fast in the 40 or the 60 yard sprints (on land), but none of his baseball team mates could beat him across the pool. I believe he got in the pool a couple of times in college?
The initial post indicates “off season training”. I believe the benefits will outweigh the negative aspects in this case. I'm just guessing here on how they conduct workouts, but the pitchers are not going to be in the pool trying to make the next Olympic swim team. So the work out should be solid consistent motion across the pool. This will balance musle groups on both sides of the shoulders, down the back and across the chest. Add the other parts of the core and legs all getting a good “LOW IMPACT” work out; I see nothing but good things here!