FYI -- My insurance company finally gave me an answer, after about 6 phone calls. The following came from the claims underwriter, who I spoke with directly so it is definitive. She agrees it is a good cause, but said it would be an excluded activity unless I purchased a charter endorsement, even though I would not be paid:

"It was a pleasure speaking with you today. You inquired as to the coverage needed should you wish to offer a 1/2 day to a full day cruise on this vessel as an auction item for the (charity name deleted). You indicated that you would be operating the vessel and there would be no paid captain or crew. Should you choose to add the Charter Endorsement to your policy for a premium of $500 that is fully earned, please contact our office. The endorsement maintains that you have a Coast Guard License. Let us know if/when you wish to proceed with this coverage."

She knows I don't have a CG license. I do have a licensed friend who would probably come along gratis, but I don't want to impose or risk making him feel awkward if he'd rather decline. The "fully earned" language means that I can't add it, then drop the endorsement for a pro-rated adjustment. IMHO -- the BASTARDS are trying to profit from other people's good-will. (I won't name the insurance company because I'm still with them and this post would be giving them bad publicity.)

I decided to give the $500 to the charity in question. It's a shame because the auction might have fetched much more, and we might have met some nice people too.