I have a friend in the club that jointly own a Hylas
47 with a partner. I used to own a duplex with a partner.
No matter what you say or think it's business. You need a contractual agreement. If there is a loan involved then I'm not sure how you can manage it contractually. Mostly it needs to be in writing and be notarized. It should spell out all the conditions including bailing out of the agreement, paying expenses, times for using the boat. Mutual approval of expenses, and anything else the parties might want to consider.
With two people it is a bit easier but with more the problem of deadlock and in fighting can happen. If the boat is US Documented you just need each signature on the paperwork and one owner to act as the agent for getting the renewal notice annually.
You could form a corporation but you will need a lot more paperwork and help filing all the annual tax forms and probably need an attorney. When I jointly owned a duplex we hired an attorney to draw up the agreement. The bottom line is it really requires a formal document but not a corporation.
advice you get on the Internet
is probably not worth the price
you pay for it.