undervoltage issue?

Rick, it sounds like the marina is using a 3 phase 208 supply to provide the two legs of power that our 50 amp cable needs. The problem with this is the you only get 208 volts between any two legs of a 3 phase supply. The equipment on the Hanse will run on 208 but it will not be happy and will be under powered. I would also imagine that the appliance outlets would only give 104 volts. Check it out next time you’re onboard. 3 phase 208 is very common in commercial supplies. However most marinas will provide a SINGLE phase 240 to the shore power outlets. What surprises me is that this has not come up a long time ago at the marina.

Mostly correct.  Single phase or split phase as you say, is actually derived from a single leg of a three phase service. A single transformer will take one leg of the three phase service and create the two legs of 120 which will also give 240 across those same two legs. Most transmission lines in the us both residential and commercial is three phase. As I mentioned before, you can’t be getting more than 105 volts at your outlets in this situation. And only 208 to everything else.