Leave in boat, hooked up to shore power to trickle charge but it`s all on a timer to go on only for a few hours a day .

When I read about boat fires more often than not it involves a dead battery and charger not playing well together. Even in sunny Cali I disconnect them and just give them a boost in the spring.

In my old boat, with lead acid batteries, I used to pull them, store them in the basement and charge monthly. They only averaged 5 years before needing replacement. My mechanic told me to leave in the boat, fully charged and disconnect the negative. There was no change in battery life. However I now have a bigger boat and the hatch is a biotch to open with no power. I also have Mercury Racing AGM group 31 batteries which cost a small fortune and weigh 50 lbs each. So I want to spare my back while keeping these batteries as healthy as possible. So, if I just turn the battery switch off, will that prevent any parasitic drain. My bilge pump is wired directly, but are there any other circuits that might draw power even with the switch off?

Be surprised the drain that can occur across the top of the battery from post to post due to crud on the surface.