Maybe answer is to form a buying group rather than an insurance company. Used to be able to do that with helath insurance before the companies broke tthe code and screwed everyone on that. Down here in Florida it was called CHPA and anyone could get health insurance at a reasonable price.
I like the thought of ti because I'm having same problem when I look at boat for live-aboard. It's every difficult to get anything written ona big boat that's over 10-15 years old--even the Hatteras 58YF I want. Brokers are saying it's a MUST to put an insurability clause in any offer submitted.
The same idea might work for a while on high performance boats as well.