When an FCC licensed amateur is operating an amateur rig aboard a US-registered vessel in international waters, he or she must follow Part 97 of the FCC rules, particularly Section 97.11. US and Canadian licensees need no special permit or authorization other than their own FCC or DOC license as long as Section 97.11 is followed and they stay within the US and International waters.
If the ship is of foreign registry, you must obtain a reciprocal operating authorization from the country of registry in addition to being in compliance with Section 97.11. When amateurs enter the territorial waters of a country, they fall under their communications jurisdiction. This means that they must obtain the required reciprocal operating authorization.