The ground that lightening seeks is outside of your
boat (the
water or the sky depending on how you view voltage potentials during the various stages of a lightening strike). You run into big trouble when you get into it's way out of the boat (looking at you,
deck stepped masts with your in
mast wiring being the most immediate path to things like bonded thru hulls, keels,
rudder posts, etc). This is typically easier to deal with in a
fiberglass boat (at least one very substantial below the waterline
thru hull needs to be metal and bonded to
mast using heavy gauge copper wire). Metal boats are an entirely different matter, but it boils down to your
electrical system negative being completely isolated from the boat
hull ground, in addition to the mast having a good lightening strike distribution system (and your battery bank should be well fused... think tesla active fuse), because no matter how good a job was done isolating battery ground from
hull ground, a strike will melt wire
insulation as a matter of thermal proximity. Fero-concrete boats are one strike away from sinking. At the end if the day lightening is similar to a flash
arc hazard. There is plenty of engenering that has gone into mitigating that hazard, so look to the utility sector for solutions. The end goal isn't to save your li-on
batteries; it is to keep your cells from rupturing and causing an internal boat fire (YouTube rossmann and battery fire to see what that can look like). Personally, I would treat li-on
batteries a lot like
propane tanks... something to be housed in a compartment that, if suffers a catastrophic failure, won't sink the boat. I believe there are a couple of cats that keep their battery banks in the outside between the hulls so that, if the batteries caught fire, could burn through
deck and drop into the
water without sinking the boat.