I would not start connecting wires or outlets until I knew exactly what is going on. US power on the boat could be hot wires of 125V each and, although one hot should be red and one black there is no certainty that the boat is wired following the color code.
Do you know if the boat is wired for US 250V which could have two separate circuits for 125V each or a combined circuit from one hot to the other of 250V?
This does bring up the question, what is your current
source of power? Generator
, invertor, shore power
? Is the boat in the US and getting shore power
? If so, a photo
of the end of the plug
or the socket on the boat would tell.