RE: Boiled Batteries?
The most likely victims of the extra voltage are light bulbs - they won't blow, but they will have their lifetimes reduced.
Folks wanting the longest light bulb life would do well to get a Graingers catalog (or similar).
Any full list of bulbs will show their designed operating voltage.
Lights that will be ON with the boat under power are best selected from the 14V list.
Bulbs that probably will be off ,,,bunk/ head reading light and most interior lights chosen from the 12v list.
A nice touch on most flying buses is a "Bulb Locker" where pushed into foam are 3 or 4 of every bulb in the cockpit , and even some galley/cabin spares.
12x12 and 3 or 4 inches deep, most any plastic box could be fitted as a bulb locker,
Screw it on a bulkhead someplace and save the hassle of looking for it.
Good place for those class T fuses for the inverter too.
A small one in the glove box of my VW towd saved a ticket, a cop stop for a bad bulb was no hassle when I changed it out instantly.