Have you investigated *ihi Bupa
International Health Insurance?
I have had their coverage for more than 15 years, and Peter was covered by them until he qualified for US Medicare. *For overseas travel he still has to find supplemental insurance since Medicare does not cover insurance expenses outside the US. *We have both been very satisfied by their response to claims. *Since I am grandfathered in, I can't say whether the exclusions you have found apply to them, but it's worth investigating.
I would seek clarification of the exclusion "anything caused directly or indirectly by the following, unless you have contacted us and we have confirmed in writing that you will be covered: *# taking part in dangerous expeditions or the crewing of a vessel outside coastal waters."
WHAT is "anything caused indirectly" by crewing of a vessel....? *If you arrived in Naples as crew of a yacht, and got hit by a car, or had appendicitis, or fell off a barstool, would that be considered to be caused indirectly by crewing on an offshore vessel? *If you had appendicitis while you were crewing on a vessel and sought medical aid as soon as you could, would they consider that medical emergency to be caused indirectly by crewing on the vessel? *
Any injuries that occur on the vessel should be covered by the boat owner's insurance, a good reason to be sure that the owner carries sufficient insurance, and that his operation is legal (i.e., he is not conducting a charter operation under the guise of taking on crew).
From my perspective, I think that any travel insurance would work, provided that you had insurance coverage while on the offshore yacht, not much different from insurance coverage for passengers in a private automobile.