It's not the GFCI tripping -- it's a cut-out due to low voltage that sags even more when a load is added. Neither the GFCI or breaker is tripping, no click from those devices -- it's just the voltage seems to cut out. I think it's just a bad connection at this point, and I suspect that if I could measure the voltage on another outlet (same circuit) while plugging something in with a load it would reveal more info (maybe that the voltage isn't "cutting out" -- it's just getting too low to operate anything including the green LED power indicator on the breaker panel). But I didn't get to that conclusion while still on the boat today..... I suspect the shore power cord has high resistance and will re-wire it, then take more measurements.