You would like to think I would have figured it out by now, to check everything, but no. On an old boat, even if it looks right, double check. I was replacing a funky 12 volt socket. It was wired with a red wire and a black wire. When I connected them to the new plug, I got nothing.

I traced down and replaced the wires---The old socket was wired with the black wire being hot and the red wire neutral......

Once I figured that out, the new socket worked fine