I don't have an ocean-going vessel, but if I did I'd buy rescue/tow insurance. It's not super expensive and when you need it, you need 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.