From the Raritan web site (FAQ section):
Raritan Engineering | Water Heaters FAQ
My heater will not heat the water on electrical hookup. What is wrong?
Have an electrician check the reset button on the water heater thermostat then for the availability of power at the connections to the water heater.
Check for power at the heating element with the tank filled with cold water and electrical power to the water heater turned on.
If power is available but the water isn't heating, the heating element has failed; if power is not available at the element, the thermostat has failed. Failure of the heating element is especially prevalent in the spring when the boat is being commissioned for the season.
Should electric power be accidentally applied to the water heater before the tank has been completely filled, the heating element will burn out in a very short time. Always be sure the tank has been filled before applying electric power.
Replacement heating elements are available from Raritan, part #WH1A-S for the screw-in element, part #WH1A for the older flanged bolt-on element. Replacement thermostats are part #WH16, for all Raritan units.