Will swimming help your pitching abilitys ?

Besides stamina, which it’s obvious it will improve, what about arm speed, velocity and power ?

I would only use the pool for joint integrity training. Equally internally and externally rotating your shoulder in the pool along with shoulder adduction and abduction will build integrity in the shoulder. Swimming is an aerobic exercise and pitching is an anaerobic exercise. This would be training outside of the realm of your sport. I would suggest you find a good strength training program that trains fast twitch muscle fiber through explosive lifts and plyometrics.