Have not done boat agreements, but did do a couple of airplane shares.
Key points: (or sharing will never come out well in the end)
A. No blame: Simply stated, one or the other of you will be a real dufus at some time. Accept that up front and just make sure that your insurance is comprehensive enough to handle all possible events, independent of fault.
B. Us only third party maintenance whom you both agree in writing on. (Owner maintenance will be a deal killer if anything goes wrong).
C. The boat is always returned after use to a mutually agreed predetermined state of cleanliness and readiness for the next user...even if the next user is the last user. Put together a check list of what is done to get the boat to that predetermined state and laminate it and keep it on the boat.
D. There will be normal wear and tear from user to user. Accept that. His scratches are your scratches so to speak. You are happy that he had enough fun to scratch the gel coat.
E. Clear method to manage when, A or B gets the boat. Lots of ways to do that using a one year calendar and going over it ahead of time. That is the best way. Then, if user A doesn't use it on his date, then user B does have the opportunity. (KEY POINT: If user A does a lot of boating and user B does little, then B cannot require user A to pony up more of the costs. You will find that one of the two users will peter out and one will be a consistent user of the boat. Use will get out of balance by and by and both users need to accept that all costs will be equal share anyway). I am not a fan of partnerships that try to develop cost per hour agreements. Even though in the airplane world that is common. I think that two partners can get along just fine in the boating world by just splitting all costs equally except for whatever it takes to have the boat ready for the next user; such as full tanks at the end of use.
F. In the dreaded, but common instance of disagreement on some issue between the users, select and name a common third party mediator up front. For example, a really smart boat shop. NOT A LAWYER! (Unless of course, party A defaults on the partnership and refuses to hand over title and or cash to remedy the default).
I'd be happy to sign a simple document if it only contained these key points without a lot of legal mumbo jumbo.
More important than the partnership agreement is a "boat operations manual." If that is followed, then the partnership agreement goes into a file and never needs to come out.
One of my best partnership agreements used a new and separate checking account. Both partners put in an identical pile of money at the beginning of the year and all boat associated costs were pulled out of that pile of money during the year for the agreed activity or use of that money. One partner paid the bills out of those funds for year one, and the other on alternate years. Asking two people to pony up their 50% on a monthly or "as needed" basis is a mistake in my opinion.
Accept the fact that a partnership is more expensive per partner than just half of being a lone boat owner. A single owner can let stuff slip around in time like maintenance, cleaning and so on. Partners can't afford that luxury. Third party maintenance, continual cleaning etc. costs more. But, that's okay.
Just my thoughts without regard to what you find on the internet.
