The UPU Letter Post Manual details the current correct treatment of postal articles mailed aboard ships on the high seas. This should answer questions as to whose stamps and what rate applies -
Quote:
Article RL 119
Prepayment and stamping of items posted on board ship
1 Items posted on board ship at the two terminal points of the voyage or at any intermediate port of call shall be prepaid by means of postage stamps and according to the rates of the country in whose waters the ship is lying.
2 If the items are posted on board on the high seas, they may be prepaid, in the absence of special agreement between the designated operators concerned, by means of the postage stamps and according to the rates of the country to which the ship appertains or is under contract. Items prepaid in this way must be handed over to the post office at the port of call as soon as possible after the arrival of the ship.
3 The stamping of items posted on ships shall be the responsibility of the postal official or the officer on board charged with the duty, or, failing those, of the post office at the port of call at which these items are handed over. In that case, the office shall impress the correspondence with its date-stamp and add the word "Navire", "Paquebot" or any other similar note.
#57454; Commentary
119.2 An application of maritime practice according to which any ship on the high seas is "part of the territory" of the country whose flag it flies.
The country issuing the postage stamps is considered as the country of origin, even when the stamps, having been affixed on the high seas, are cancelled at the next port with the date-stamp of another country.