The boat is in salt air so the pin connectors is a really good one to check.The other thing to check is the pin connectors, especially the ones for the power wires.
(don't ask me why I think that)
Also, there is some good grease one can put on the connectors to help prevent oxidation. One type for when there is no chance of shorting nearby, and a different one for when there is a chance. Unfortunately, don't have the names at my fingertips. I *think* Oxguard(sp?) was one of those I've used (ye old memory isn't what it used to be), and don't remember the other one.
One really needs a volt-ohm meter to troubleshoot some of this stuff. Another person and some long leads are also helpful. Even a cheap Harbor Freight VOM would be better than nothing. Got one for free several years ago so just leave it on the boat; The Fluke I keep at home.