Just looking through Bulbapedia and I found this:
Apologies with the size, I couldn't find a bigger picture.
Anyway, I just noticed something about Blastoise's cannons. If you look carefully, you can see that the white part(nozzle) is seperated from Blastoise's flesh(the blue part).
In my opinion, that nozzle is actually an artifical construct given to Blastoise to concentrate its water. I don't think Blastoise recieves it in the process of evolution but is instead bought, maybe from a Pokemart.
What I'd really like to see is a drawing of Blastoise without the nozzle. Thoughts?