I am setting up a boat sharing partnership and am wondering how the insurance will work. The situation is basically that I own the boat and will have up to 3 other partners who will pay a monthly payment and also share certain costs - as I understand it, this will essentially be a bareboat charter. Can I use the same insurance I now have and just add the partners as additional insureds to my insurance or will I need to have commercial insurance (which would be prohibitively expensive) to make this work.

You haven't said anything about how much money is involved, but I don't see why a joint venture agreement wouldn't work. The venture would have a name and the agreement would set out what each individual participant's duties and rights are. The insurance could remain in your name. Here is a sample JV agreement, which is far more complex than what you need, but you'll see what one looks like.

http://www.nist.gov/tip/...venture_agree_032010.pdf 

You should be able to add their names to your insurance. It is done by an endorsement to your policy that the agent can take care of it. It shouldn't matter that they are paying you, etc. and you shouldn't need a "commercial" insurance policy which would be very expensive.