In looking through the postings, I realize that it's a little late to be responding to a thread from last fall, but there [i]is a way around this type of situation. The problem with residential water heaters is that the thermostat on them isn't ignition-protected. So each time it makes and breaks to turn the electric power on or off, it makes a spark. If there are gasoline fumes present below deck, then "Boom!" This is only a suggestion on how to replace the thermostat with one that is ignition-protected, to remove the danger of blowing up the boat.

Raritan Engineering offers an ignition-protected water heater thermostat, Raritan part # WH16, that slips into the same spring clips that are used to hold in the thermostat on most residential water heaters. It ain't cheap ($89.60 factory retail price), but it is still cheaper than purchasing a whole new water heater - and it's definitely cheaper than taking a chance on blowing up the boat each time the heating element is turned on or off!.