As others mentioned maybe the target dodged the actual water and got injured by something else is one way to handle it.
Another way is to say that since it's conjured water it disappears immediately after the attack as it turns back to ectoplasm.
If he wants water that sticks around longer he could manipulate water in the atmosphere, but that's a maneuver so he can't do any actual damage on that turn, afterwards sure.
Since it's a maneuver that means 3 shifts of power minimum against one target. Against the floor that's a maneuver on the zone, so minimum of 5 shifts. If someone manages to dodge and get out of the zone then he's not going to be affected by whatever goes down in the zone he left. If he doesn't get out of the zone then he's a valid target for whatever is done with the water in the zone next.
If your player wants to freeze a target and make the floor slick, then that's two maneuvers I think, so minimum of 6 shifts. Think of it like this, the water is on two targets, the floor and the enemy. He can do both, but due to the mobility of the enemy it's going to take more power and concentration.
I don't know how other people feel about it, but if you really feel you can't justify not leaving aspects on a target hit with a water attack then just up the power cost of the attack, however many shifts of power are put into the attack, and at least 3 shifts for the maneuver that leaves the aspect. Really you're probably not supposed to be able to maneuver and attack at the same time, but if you keep upping power costs so he risks backlash, fallout or consequences it should balance out. I'm not saying punish the player for being creative, but you can totally point out that he's trying to concentrate on doing multiple things at once while he plays Xanatos Speed Chess on the battlefield.
You could also charge shifts for duration (i.e. the water sticking around), and probably should.
I know someone raised the point that conjured fire sticks around and acts just like normal fire after it's been conjured. I would like to point out the difference between conjured water and conjured fire. Conjured fire burns stuff, this generates real fires because of heat and combustion principles. The conjured fire probably goes away almost immediately and leaves vanilla fire, fire makes more fire. Water on the other hand is conjured matter instead of energy the way it's being talked about here, so it's more likely to turn to ectoplasm and simply evaporate after an attack before it can be used for anything else. If it's been obtained via a maneuver (sucking moisture out of the atmosphere) which is a turn in itself it's not likely to stick around and behave exactly like normal water. If it's been obtained via the conjuration method then it's probably going to require shifts in duration for it to stick around long enough to affect subsequent turns.
