I cut off the ends and re-did the connections in the cable. Now it tests zero ohms round-trip with clip-on test leads shorting the prongs at one end.

I made a quick stop at the boat and plugged in with the re-done shore power cable. Everything stayed "up". I would say I think the problem is solved but there is still a voltage sag under load... I plugged in an electric heater (1500w) and measured the voltage in the other receptacle on the same twin outlet:

1) Fan only -- 121v (same as yard outlet)
2) low heat -- 108v
3) high heat -- 105v

The good news is the heater didn't shut down, the the breaker/GFCI didn't trip and the green power indicator light on the breaker panel stayed lit -- but now I'm thinking maybe the outlet in the boat yard is marginal (it is outdoors against a fence). I didn't have allot of time to stay there, but next time I go back I'll plug the heater right into the boat yard outlet (without going through my boat's shore power) and take measurements on another receptacle in the same quad outlet. If my hunch is right there were two additive problems and I've solved one of them.