cried a little and called the charter company. Spent the day swimming at the anchorage waiting for them to show up with a new charged battery (the old ones were so discharged they couldn't even be charged by a fresh one)
Dude rented a charter. It would be pretty silly of the charter company not to have some kind of "rescue" provision, wouldn't it?
We had a very similar story, except we were the first to rent the boat at the start of the season and the battery bank hadn't been very well tested before they gave it to us -- a few had gone off over the winter. They drove out from HQ, replaced the bad ones with fresh stock from their van, and sent us on our way.