Without a doubt this is 100% a risk that should be placed at Lloyd's of London as LadyBroker says. I used to do a fair amount of work with those guys over then when I used to work there.......it really is the best place for that kind of risk.

If this is a small personal pleasure craft, sometimes you might be able to endorse to the craft onto the owners current homeowners policy...granted its value and speed of travel are both low. The other recomendations left in the response above are good ones.

Which markets have you tried? Have you tried to get temporary coverage for these parties endorsed to the individual vessel's policies or are you looking for stand alone LLLL and GL Coverage for these parties?