RE: Looks like I have avoided a fire
110V 50A was the way power was supplied in the 50's and 60's , very rare to find any location with this socket.
I would use a 50A 240 power hose from dock to boat inlet , and use CB from there to protect the wiring.
That way for most boats should a plug go south the unit will probably not burn.
Remember cheap GRP burns at 500% of oak , that's why Fire Retarding resin is demanded for boats that carry ( over 6) passengers for hire.