There should be a shutoff...that's on the cold line. Occasionally, they put a shutoff on both sides, but it is much more common to only have one on the cold side. If the hot and cold are reversed, you won't get as much hot water out of the tank since you'll be drawing water from the bottom of the tank rather than the top. Normally, you pull it off the top, and there's a second heating element up there to aid in keeping that last bit hot. Also, if it came with heat traps, they are only designed to flow one direction, and while they may not stop the flow, they'll reduce it.