I had a boat partner (A friend) on my Hunter 280 and then my O’Day 322. Both boats valued under $30k.
We registered the boat as co-owners, and had a 1-page agreement (not even sure if we sighed it...but we used it to go over expectations) that outlined how we were sharing costs, agreeing to upgrades, and what happened if one of the owners wanted out of the partnership.
We shared routine costs 50/50, and shared the boat the same. We ever had an issue with scheduling. If we needed to replace something (we replaced VHF, bought a Mack Pack and a winter cover, etc.) and discussed before we spent any money.
Eventually my friend wanted out....and I had to decide to either keep the boat and pay him 50% of the purchase price (we did not add in the upgrade costs) or we would sell the boat and split whatever proceeds we got.
I debated using the opportunity to sell, and “upgrade”, but I like the boat, and didn’t really see much in the same price range I would rather have. So I bought out his share and it was done.
The funniest part of the story is that while we co-owned the Hunter, my dream boat came available. My wife was dead set against buying it until we sold the Hunter...but my partner and his wife were all in favor...so we out voted my wife. She forgave me a few months later when I sold the Hunter

. Without the partnership, I may never have got my dream boat!
We did not have a loan, which made things easier. And we are still friends!
I guess we got lucky.
Greg