I was in a boatshare with Intrepid until a little while ago. The problem we had was that I wanted to use the boat more than the partner.. therefore I was clocking up the hours, and when the time came to service it, I ended up paying the service, because he'd put 10 hours on to my 100 hours (or something like that)... Also, when there was work to be done on it, I tended to end up paying for the work to be done, as I used it most...
The boat was quite expensive to run when I only had access to it 50% of the time, the rest of the time, it would just sit in the marina, rarely being used by the other partner... and when I asked if I could use it on the lovely summer days ... it was normally "I might use it later..."
I am now in another boat share - but its a much smaller boat, and a lot more manageable... and we are normally both be out with it at the same time, so doesnt cause any issues. Its a small engine, so parts are reasonably cheap (compared to intrepid!), and most of the maintenance we can do ourselves, and the best bit... is that it is small enough to live in my garage so we don't have to pay expensive mooring charges
So to summarise, I think boat sharing can work, but it depends on the people, and also about the amount of money the boat costs to "run/maintain"