You would use either the physical presence or Bona Fide Resident test whichever applies. The country you located in is the foreign country under which the vessel was registered.
Per Bebb (1961) the US Tax Court upheld this notion:
"It is petitioner's contention that being aboard a foreign vessel as a member of that vessel's crew is tantamount to being in the foreign country under which the vessel was registered, since petitioner was subject to the laws of that foreign nation and since he was traveling over international waters. Petitioner contends he was, therefore, present in a foreign country as required by the statute. Petitioner contends that the 510-day provision of the current statute is a modification of predecessor statutes and that Congress intended that physical absence from the United States for the required time be sufficient to bring individuals within the statute."
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