I'm usually the sole person in the crowd saying "partnerships can work", mostly because I've had positive experiences with them in the past (with airplanes though, not boats). In this case though, I think you are expecting a bit much. The short of it is, IMO you'd be better off buying the boat on a loan, finding 20 people that want to rent it regularly, and rent it out to pay off the loan.
I think you'll be hard pressed to find 20-25 people that want to buy into a boat that they have to compete against 20-25 other people to use. They'd be better off taking their investment and renting someone else's boat, the price would be about the same, they wouldn't have to worry about recurring expenses, and scheduling would probably be easier. You run into all sorts of issues when people put a financial stake into something and they can never use it on their own, not to mention the resource management problems you come into when you have 5 "owners" on the boat all wanting to operate it differently.
Also, keep in mind that when partnerships run into roadblocks, whether with 2 people or 50, it's the shared assets that suffer. I can take you to our local airport or marina and show you plenty of boats and airplanes rotting because their owners couldn't agree or get along. The chances of this type of situation happening increase with the number of people in the partnership, as finding that many people who are willing to agree on how things should be done becomes more difficult.
I should also point out that on more expensive boats, repairs
and parts are more expensive. If you buy a $100k boat with a $4k buy-in, you are going to eventually run into problems where repairs
have to happen, but the owners got in over their heads and can't afford it. The effect of this is compounded if the owners haven't been able to use the boat they want to.
In my situation, I had a group of 4 partners who owned an airplane. We all agreed on how things should be done, we worked well together, our scheduling went great, and we generally enjoyed being around each other. We had awkward moments and disagreements, but the partnership succeeded because we had a strong enough relationship to make it work. We knew each other beforehand and spent a lot of time nurturing that relationship though. There is no way you'd accomplish that with 20-25 people.
Just my advice. I'd sell 1/2 my boat in a heartbeat if I knew someone that would buy it, not for the money but for the relationship that comes out of it, but these things get tricky and they get trickier with more money and more people.