No I don't fail. I just bought the water pistol and tested it out on crabs: does 5 damage. Then I tested it out on a player and his health dropped 6 damage. I also bought the water gun and tested that out. 5 damage on crabs, 6 damage on players. If you don't believe me, try it out on your own. I'm not sure how this is possible unless when you buy the water gun/pistol, it's already upgraded.

BTW, where do you upgrade/repair the water pistol and water gun?