Start here? This from your post #15: Again I am hesitant to get too granular pondering the numerous little gremlins on a boat, but in the last year I did have problems with several electric tooth brushes charged on both 12v & 120v. Never had a problem with them for several years previously. Finally gave up buying new ones. No obvious problems with other devices plugged in to outlets.
Sounds like something has changed if you are unable to make use of something that worked fine previously. Maybe a combination of an electrical device failing and inconsistent zinc wastage due to other wiring problems that cause inconsistent effects.
Possibly this? This from your first post: I put new zincs on my trawler shafts and in couple months 1 is 1/3rd dissolved (my zincs used to last a really long time), and its active enough that it is the one place underwater that has no growth. I even saw very faint bubbling from that zinc. Other zinc is fine.
Seems like you need to have a look at the drive train on that side. Try disconnecting the engine from the batteries entirely including the alternator if it runs to the batteries separately to see if the fizzing stops. Don't rule out the engine harness to the instruments and controls. If the fizzing stops figure out where the leakage is coming from. Alternators have been known to do that. If it stops, you may be able to begin reconnecting but checking for voltage between the engine and the connection. A voltage or sparking would be clues when you make/break connections.
Make sure you are not using the engine as a DC negative buss bar. Better to move those connections to a separate buss bar with a direct path back to the negative buss.