After I turn my central air conditioner off, a pool of water collects on the floor. What could the problem be?

Where does it collect on the floor, under the A/C unit? In the center of the room? In another room?

Oddly enough, it seems to pool around the water heater or so it seems.

Oddly enough, it seems to pool around the water heater or so it seems.

Likely because that is a low spot on the floor nearby. Does your Central air have a condensate drain run outside or to an inside drain? Does it have a pump that moves that water to a drain further away? I assume from your post that water never collects when the unit is left running, only when you shut it off.

It only happens when the unit is off. I don't know about a pump, but the drain runs runs to another drain under the floor. Any thoughts?

It only happens when the unit is off. I don't know about a pump, but the drain runs runs to another drain under the floor. Any thoughts?

What you described is a typical setup. I'm trying to think of something that could cause what you are seeing, and only under the conditions you see it.