Well, for the Water Gun-thingie, if there is to be any kind of balance in nature then that water can't simply appear out of nowhere, (As that, considering the sheer amount of water pok?mon in existence, would surely lead to the whole world getting flooded in the long run, good for Team Aqua I suppose. XD) Which leads me to agree with Dark Weezing here and say that it's basically just releasing excess fluids onto your opponent (Or in laymen's terms, spitting), although considering the frequency with which they do so water pok?mon would probably have to store up a lot more liquid than they actually need to survive, just in case they need it for ammunition.