You can definitely re-fill the gallon jugs along the way here and there. So, I usually get about 12 gallons jugs (2 six gallon cases from Riteway) for 8 people for 10 days. I've ordered three cases before (18 gal), but it was too many jugs and a waste of plastic. You can also pick up a another gallon jug at the small markets too, if the 12 gallons is not enough between refills.

In all likelihood the boat tank water is just fine too. Charter boats are used quite a lot and therefore usually have fairly fresh water in the tanks. I've drank the tank water many times (rather than bother to get out a jug) with no adverse effects. If you end up needing some tank water for drinking, it's not anything to worry about really.