... You know as well as I do that
the boat can and will stay out for more than a day.

It's a planning boat with jet drives. You think it's going to cross
oceans?

....
AGM batteries are good but they are overpriced by at least 4X. You
could put together a lithium ion bank for the same price that would last
longer, hold more charge and charge more quickly . . .

AGMs are about $2/AH at 12 volts. I don't think you can get lithium
ion for anything close to that. I've recently paid about an order of
magnitude more on an amp hour basis for computer batteries at deep
discount. Lithium ion batteries are tricky to charge and a fire
hazard if not very carefully controlled, but if the price were
competitive lots of folks would be using them...

-- Tom.