What's Inside a Six Gallon Marine Water Heater? Not much. Not shown is the rectangular aluminum box and insulation that wrapped this pressure vessel. It was not winterized properly and freezing water burst the vessel at the weld. About half the weld was broken and I ground through the other half, the aluminum was really thick and it took some effort.
The closed loop is where the hoses carrying engine coolant connect. The other two pipe stubs inside are in the input and output of the potable water system. The long skinny loop part is the 120V heating element and the rest is an overpressure valve and thermostat components.