what exactly they're insuring against.... someone getting injured and suing the Seasteading Institute?
I think so. I doubt the underwriters took the ideology into much account, they probably just went with the standard factor for P(lawsuit | serious injury) which I would guess is over 50%.
The Seasteading Institute had to cancel their annual on-water get-together, Ephemerisle, after being quoted ridiculous insurance costs, and it came back as the unofficial Floating Festival - no tickets, no organizers, you just show up with a boat. (Or find someone else who already has or is renting a boat and still has a spare spot, etc.) Jul 22-25 with an unconference (i.e., show up and give a talk) on Saturday the 24th. Posting here because of the large de facto overlap in the communities. The location is about a two-hour drive from the Bay Area.
Main info page.