It doesn't seem unreasonable that this is something that would be extraordinarily expensive to insure -- and the insurance cost balloon wasn't 10x, but 2.5x. I mean, there are going to be drunk/high people jumping from boat to boat and doing various motorized water sports.
I'm not entirely clear on what exactly they're insuring against.... someone getting injured and suing the Seasteading Institute? I wonder if the insurer factored in the fact that everyone involved in the event believes in a political ideology of extreme personal responsibility, and are therefore much less likely than the average american to sue some random third party when they slip and fall.
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.