I have an AO Smith FPSH 40 power vented water heater that performs quite well and is fairly efficient. The power for the vent is supplied from a line cord and a GFCI-protected outlet. Is there any reason NOT to install a digital timer switch (the one I have is rated for 10 amps, several times the draw of the fan) so the unit shuts down at night rather than keeping the water hot? FWIW the unit has no standing pilot and the controller is rigged so that the gas supply shuts off if the power is cut, so there's no probably if it's heating when the switch turns off...the same thing can be done with the manual switch on the side of the blower unit in fact.
thanks,
Kiwanda