Only if you count a dead battery as a problem
The water heater will try to light a few times and should stop. If it re-tries it will just give up again. The heater is a 12 VDC device, right? If it is a 120 Volt device it will have been shut off when you turned off the power.
Absolute worst case, you could burn out the ignightor system, but I doubt it.
I just re-read your post. The univolt will keep the battery up, so the water heater will just keep retrying. I do not know how long it tries before giving up.