@Patricia1771 Contact the lawyer involved and request a copy of the required document. Have your own lawyer look at it to ensure that it is binding on the guest. You should also get in touch with your insurance company and find out what liability you will have if your guests are injured while on the lake. Does the lake owner provide any insurance? You mentioned a boat -- is it properly insured for use by guests?
Be sure to include in your listing that use of the lake will require signing "name of document". Once the guest has booked, you can send him a PDF of the document that he can print out, sign, scan and email back to you. Once you have it back, you can print it, sign it and send it to the lake owner or his attorney.