In my experience, a healthy lead-acid battery that is fully charged by a known-good charger
just before disconnecting all loads can be left disconnected for 6 months with no ill effects or loss of battery life.
The best possible arrangement would be to have some someone check the boat
once a month while it's stored, and measure the battery voltages. Maybe put a charger
on them for a couple of hours, then leave alone for another month.
Leaving some sort of trickle charger (solar or otherwise) on them permanently might not be the best option; they are devices that could fail, and if they're not intelligent or well-regulated, they could potentially overcharge the batteries