It's not really a bad idea. The obvious benefit is that it would definitely save time, and since other classes have gotten timesaving skills like this, pretty much the only thing anyone waits for in a raid now is for the mages to make all the water they need to give out, and then drink again to regain all that mana. The only real argument against it is that unlike, say, Ritual of Souls, the mage water spell doesn't require any costs or reagents-- just mana. So the devs might counter that Ritual of Souls was meant to make the reagent cost cheaper for warlocks, but mages are supposed to use their mana-- that's how you play the game.
I guess you could make a parallel with buffs as well-- if we're not waiting for mages to drink after conjuring their water, we're waiting for priests to drink after rebuffing all the time. The only way to keep that time from being wasted is to give the priests mana-free buffs, and that ain't going to happen, so maybe the devs aren't hot on the water well idea, either.
But considering Ritual of Souls was implemented, a food and water well doesn't seem too far off. Maybe there'll be an extra mage quest in the next content patch?