This might be an opportunity for the kind of nuclear 'insurance policy'
that Erwin Strauss wrote about in 'Basement Nukes'; someone sets up a
nuclear-armed insurance organisation and the ship-owners pay an annual
fee so that if a nation does attack them, the insurance company will nuke
the capital of the attacking nation.
Obviously they must be decentralized enough that the nation can't nuke
them back, but that's not too hard compared to the problems of defending
a nation (see the difficulty of finding and destroying Scuds in Iraq).
Mark