Its a 30 A breaker and an ELCI or a GFCI. It is protecting the inlet power to the boat.
I'm pretty sure all ELCIs are 2 pole breakers because they must break both the Hot (black) and Neutral(white).
They must be within 10 ft of the shore power plug.
The OP picture looks like a single pole breaker but it has a TEST button. Thats confusing me.
The Galvanic Isolator only connects to the inlet Green ground wire.