Hey, I'm sure we have ALL bought minecraft singleplayer AND multiplayer. We did not buy restricted access to parts of the game, now did we? I'm sure if I made a game with multiplayer I would make a rule for serverowners that ALL parts of my game are allowed! [EDIT] Besides, if limit water source, why not limit all blocks?
Lots of people do water jobs for free already anyways. Be social and post in trade chat, it's not that hard