That was exactly the argument
They contended that while we were in possession of the boat, it was "as if we were the owners, and the security deposit was the equivalent of the deductable" ($750 or so, if I remember correctly). The loss, they said, was the owner's loss, and since we were the owners during that time... Other charter operators I've spoken to since have said that either their insurance would cover such a loss, or they would out of pocket. The lesson for chartering may be to either have something like this clearly stated, although I don't think you can think of every contingency, or to work with a charter who offers insurance, usually something like $25/day, instead of asking for a deposit. Maybe this winds up a little more expensive, but no nighmare unexpected expenses.