Kind of a wild (read half thought out) idea. But if you make up a set of Really long leads for your multimeter, then you can test all sorts of things. After, of course, testing the resistance & voltage drops in the new leads.
Such as checking for voltage differentials between; various batteries & the water
in various spots around your boat. Ditto vs. a few spots on your engines, as well as your drives.
Pretty much checking, methodically, with intent, & in order of likelyhood of finding problems(s), everything that you can think of, that might be out of whack with your boat's electrical system
. Ditto on what might be awry in the marina's electrical
system (near your boat), within reason.
Also, of course, document all of this. In addition to time(s) of each measurement, what's switched on or off; on the boat, on your neighbor's boats (within 20-50m), on the dock, etc.
That, & ask the local live aboards, if they've noticed any changes in their zincs, & relavent systems in the last few months, to years, too.
Heck, maybe even put all of your recorded electrical
data into a spread sheet. And even if it doesn't help you, giving a copy of it to an electrically savvy boat guy, might assist him in tracking down the problem, if you can't.
Were it me, aside from making an obvious list of things to check with the meter, I'd also research
it via the internet
, & in Nigel Calder's (& similar) works.
You might get lucky, & for the cost of some wire, & a notepad... find the likely culprit.