I have been in two shared ownership arrangements. The first was a wooden 22' historical double ended launch. The current one is a 24' pontoon boat. Both worked OK but not without some annoyances. First, it must be jointly owned. We formed an association. Second, be sure you are able to secure insurance for the association; individual isn't good enough. Third, establish a share value, how it will be held, rights of survivorship, saleability, etc. Fourth, establish a reservation system or use calendar. Fifth, establish a maintenance schedule. To all that, add bank account, authorization levels etc. Sounds complicated, but its necessary if you want to worth through the inevitable issues. Obviously, you need bylaws, meetings, and those governance vehicles too.