Boat hot water heaters generally use engine coolant
, electricity or both to warm the water.
Trace out your potable water system on paper.
Normally the hot and cold water come from the same source pressurized by the same pump. The only difference is at the "Y" or "T" fitting, one hose goes directly to your faucets and the other hose goes to a heat source and then to your faucets.
It may be something like a valve along the hot water line that is turned off. If so, be careful, it may have been turned off because it is leaking or because of some other problem.