Without any numbers? You just buy as much battery as you can physically fit in the boat, all same size, same lot. In the odd event that's more than you need, that's OK too, because your batteries will last longer if you don't cycle them deeply. You gain battery life if you only discharge them 30% instead of the more common 50%, enough to more than pay for the extra money
spent on batteries.
"more battery later" is a bad idea, you'll have mismatched batteries in the bank, and that's never as good as having them all matched up front.
And on a 37' boat, you won't have enough room to get in trouble as long as you stick to big batteries.<G>
Then you buy whatever charger matches the battery capacity and run your Honda as much as you need to.
Not a technically perfect way to look at things, but very likely to be very close to what actually is ideal for you.