GFI requires either a GFI breaker or that the first outlet in a circuit is a GFI, which then protects outlets downline. Outlet is less than half the price of the breaker so that's the most common configuration I've found on boats. Also code requires a master breaker for all A/C circuits in close proximity (6-10 ft. ?) to where shorepower enters the boat. If all circuits dead (water heater, charger, outlets) stick your head in behind where the shorepower enters and follow the wire.
Also as Joe suggests start at the pedestal and trace the power from there.
Friends called me to their boat once because they awoke to dead batteries and alarms, no shorepower and they couldn't find the problem. After a quick check inside I went to the pedestal and the damn thing was melted where it had caught fire and burned itself out without anyone noticing.