That electrical tape plus the corrugated wire cover aren't good to see, someone has been in there doing something. Sounds like it's time to do some serious electrical troubleshooting and repair. Personally, I solder any and all wiring connections/splices and prefer ring connectors and screw terminals, and dielectric grease. (Army taught me all of that) I'm betting that you will find somewhere some green corroded copper with poor if not iffy contact, plugs not making good contact.
If you need to rewire and make an adapter now would be the time. Having your boat die in the middle of the lake at 2am or even worse 20 miles offshore wouldn't be the best experience. That's why for MY peace of mind I completely rewired my boat and replaced . . . . . . . are ya ready for it . . . . . . cloth wrapped wiring original to the boat