Talk to Craig Schneider at Bruce Schneider Associates631.698.3558.
I recommend you establish an LLC. The Charity then contracts the LLC. Any and all sponsors should be named insured on the policy as well. Liability can extend directly to them.
The hard part is understanding and complying with the warratny provisions of your policy.
I would not conduct any event without proper insurance.
You can find more info on the BeSafe site as well.
http://www.boatingsafe.org/