Probably this has been tried before, with disasterous results ... but I was thinking, "It's a shame batteries are so heavy, so cruisers have to get by with undersized power storage. All that lead. Sigh."
Hey, wait a minute -- sailboats are FULL of bloody lead. It's called ballast, right? So why not make a nice deep bilge,
line it with polycarbonate, hang lead plates with separators, and pour in a hundred gallons of sulfuric acid? The plates may need to be shaped to fit the bilge/keel, and you'd want baffles & restrainers to stop em flapping around too much, and they'd have to be modular so you could yank out a dead cell at need, and you'd need a good sealing cover with venting....
But youze is smart people. Let's see if you can think of ways to make this idea practical! I mean, if yer gonna schlep three tons of lead all over the place, it may as well be working for you, right?