You sound frustrated that you did not get lots of answers about this. Problem here that very few people on this forum charter boats. They all own them. You are asking about a different kind of insurance than they carry so it is hard to answer.

When I went to France for the Worlds I chartered a boat. I tried to get insurance here, from my carrier, and could not. I did get a quote for "out of the country" insurance of about $500 for a week of sailing from another vendor. I told the organizers in France about this and they we able to set up insurance for me at $10 a day. They had many people contact them about the same thing.

If we have an event in the US that is a borrowed boat situation then the event usually takes care of those details.

Later,
Dan