
Im not a kinesiologist and I have no scientific proof of anything in this regard.
Having said that, I was however a competitive swimmer, from age 7 until 14..... as in get up at 05:00 (all winter) and swim 2 miles before breakfast, or throwing up.
I wasn't a pitcher, and have no reliable information as to how this may affect a pitcher's arm health, but when I gave up swimming to concentrate on baseball during my freshman year in HS, my deltoids were much, much more developed than any of my baseball playing friends. (As catcher, I never had any arm troubles.)
I do know that the deltoid muscle set was the one emphasized in my specialty (free-style).