Eeek! I don't know, but it sure looks like it's purpose is to blow your boat up! :smt001
Seriously, though, I do not know. Can you follow the cable? Obviously it's there to protect the wires/cables going to a certain "thing" - follow the wire to that "thing"? But, I don't know if it's original or owner added, but it really doesn't look safe. See what it goes to and seriously consider replacing it with a proper, gas-engine room approved breaker (ignition protected).
