I doubt you'll have much luck going after the rental company. I used to work for a rental company, and the renter signed contracts stating they had full liability once they left the dock until they returned unless the incident was due to a mechanical malfunction of the boat. It also stated very clearly that the renter was not to operate the boat after sunset. I imagine they have covered all these bases as well. Good luck, i've seen lots of renter related accidents and they are never easy to deal with. My only advice is to get a better lawyer, you need a pit bull for this.