Roger Long wrote:

I know it's theoretically better to keep batteries on a trickle charge
and warm over the winter but it costs me $70 to have them taken out of
the boat and stored by the yard. (Nevermind the reasons why I don't
want to take them out myself and home to my basement.)

....
I would vote in the "no-problem" camp. The self discharge rate of
AGM's is very low, and in the winter temps should be a quarter of the
summer rate, or less. If you're able to visit the boat a few times,
you should check the voltage to make sure nothing as gone awry. And a
small solar panel might not be a bad investment

In my case, I have a solar panels (150W) under the shrinkwrap that
keeps a small charge on the large flooded house bank. The AGM starter
bats (Optima red top) can fend for themselves.