Not a boat but a pickup that I used for a cabover camper. I had a battery isolator that had the high CCA battery connected to just the starter and a deep cycle battery connected to fuse box and thus to everything else. I had both an ampmeter and a voltmeter that was on the line from the alternator and into the isolator. Using two batteries made both of them last a long time. I also had a tiny jumper cable in case I ever ran the deep cycle down to the point where it wouldn’t trip the solenoid.

We all like to think of ourselves as rugged individualists. But when push comes to shove most of us are sheep who do what we are told. Worst of all, a lot of us become unpaid agents of whoever is controlling the agenda by enforcing the current dogma on the few rugged individualists who actually exist.