The cost of insurance would be quite substancial. boating is not a small risk then you add in that the renter probably doesn't own their own boat so they are a "new captain" which adds risk.
if you move forward part of your partnership agreement should include who will hold the liability. you or your neighbor. Where the boat is stored on your property that could raise your stake in the "who is liable game"
liability waivers are a joke and do not hold up in court.
Auto accidents where someone dies cost in excess of $2,000,000 so if you have less coverage than this it would come out of pocket, boating accidents are much more likely to result in death. The #1 cause of deaths in a boating accident are not wearing a life jacket.
people love to drink alcohol on boats.
I am just compiling this list as a way to give you some material to think on not trying to persuade you one way or another.
I am currently renting out 2 boats. It is a side business that I have partnered with a friend on. We rent the boats with a captain, and meet the renters at the lake, take them out, teach the participants to wake surf, wakeboard, kneeboard, or go tubing. It has been a really good business but we maintain control while renters are on the boats and can shut down anything that add too much excessive liability. plus where we are providing the captain we are in total control of the boat.