Shared two boats in the past.
First one rebuilt a 21 ft jet deep v something, fibreglass, respray, new windshield, upholstery, motor and Jet. Looked great, was crap. Bought boat and co - owner announced he was off to Australia for three months. By time he got back I had pretty much finished it. I put petrol in he used it up. I cleaned it, he didn't, it sank, I sold it and gave him 1/2. Who's the mug?
Second was diesel Monterey. Lovely boat, I used it lots, he (different one) hardly did, always moaned about servicing cost, storage, repairs. Had some weird ideas about tying it up, could not get the cover right, was never as clean as I left it. I bought him out in the end.
So sharing is a great idea if you share with me, the boat will be serviced, clean, fuelled up, maintained, cover on securely and tied up properly! And I will do most of the work and pay more than my share, cos that's how I like it!
OCD, who me! Don't do it!