About ten years ago peer to peer boat rental networks started popping up. Everyone wants to be the Airbnb of something. And privately owned boats are a logical choice. Most boats are idle most of the time (according to GetMyBoat the average boat is used only 8% of the time), and boat ownership is out of reach or just unpractical for many potential boaters.
The idea is simple, boat owners can list their watercraft on a website with a price per hour or day, and renters can search options and complete a transaction to borrow a boat just like they would with lodging (AirBnb) or car rental (Turo). Many of these networks include both chartered and bare boat options. The chartered boats include a captain and are basically a private tour.
Over the past few years this business space has shifted. About half the networks have shut down or pivoted to something very different. The ones that remain have a heavy emphasis on charters with captains and custom luxury experiences. As happens with this sort of network, a lot of the listings are not from individuals. Just like Airbnb sees lots of professional property manager listings, and peer to peer tour websites end up with professional tour listing, these boat rental networks have a few people or companies with lots of listings. Not exactly peer to peer, strictly speaking. But they end up being a convenient amalgamation of options for boat rentals.
A few of the major players in this field include Sailo, Boatsetter, GetMyBoat, ClickandBoat, and BorrowaBoat.
Some of these boat networks are providing insurance to both owners and renters while others put that burden on the users. Be sure to understand your insurance options and coverage. Boat owners in particular need to make sure they are adequately covered. For bareboat rentals, networks mostly leave it up to boat owners to determine skills and certifications required by renters.
Prices are not cheap, but for vacationers who might pay for a boat trip for a day from the local tourist pier, this is a good alternative resource. Especially for fishers or sailers who are serious about boating, these networks open up a lot of options. 
As someone who only knows how to operate a kayak, I’m not likely to rent a boat myself. But a group charter for a special occasion could be a fun option.
To see a list of the boat rental websites and what regions of the world they cover select the Boat category in my Peer to Peer Travel Networks Spreadsheet.