Won't comment on hiring a lawyer (although paying a lawyer is definitely the highest and best use of money
) but would suggest that whatever agreement you come up with you run by your insurance company. Some/many/almost all of the marine insurance companies will cover "contractual liability" for the damage your boat causes to the dock. However unless you have your insurance company review the liability you have assumed in the lease (contractual liability) you won't know whether the insurance company will cover the damage.