Let me preface by stating I'm not a lawyer, but I know of a similar case from the experience of a friend:

IF the mooring owner said it would be OK for a 45 foot boat then yes, I think the mooring owner has at least some great degree of responsibility for the bad thing that happened *if* it was not fit for that purpose.

OTOH, if the agreement contained disclaimers for the renter to use "at your own risk", or something to that effect, then maybe not.

In my friend's case the mooring rental agreement contained such disclaimers but the mooring was required (by town regulations) to be inspected and maintained. The owner could not prove it was inspected and expert opinion was that it hadn't been maintained according to the standards required by local law, based on the condition of the failed shackle/links. The image below was the actual failure that caused my friend's boat to go aground (it suffered about $60k in damages). The case was settled out of court and my friend was paid for the (almost) full damages.

Would you say the mooring was properly maintained?

I rest my case, your honor. Lesson learned -- dive your own mooring or hire someone independent to inspect.