OK. The keel
box with battery
boxes idea is appealing. All batteries
should be in acid proof boxes. Add some baking soda in the bottom of each to neutralize spills. Batteries
vent hydrogen when they overcharge. Use a good automatic charger/maintainer, and realize that hydrogen, having very low density, will come on up and dissipate pretty easily with reasonable ventilation. Just don't block its route
to dissipation or expose it to sparks near the batteries.
But, a concern I've had for my house bank, which is under the floor of my main cabin
and about as low as possible in my boat
: That's the first place to flood with sea water
when I have an OH S@#T moment. My answer is a rotary switch that isolates the instrument panel and radios, with a small battery
that normally is part of the house bank hidden behind the panel in the wheelhouse. That way I will still have a battery to run the radios and call for help.
While you're moving stuff around, maybe you can make other changes to eliminate your list. Good luck with it.