Legality of gay marriage conducted on a ship?

I can’t believe this hasn’t come up IRL yet, especially since gay cruises already exist. What does the SD legal community think about the scope and limitations of any such unions officiated or solemnized by a ship’s captain, clerical figure, Elvis impersonator, etc.? Would such distinctions as the nationality of the ship’s registry or whether the ceremony was performed in domestic vs. international waters matter?

The issue of whether captains can even perform hetero marriages is a big enough can of worms without introducing more variables. See Cecil’s column for details.

Ships and whatnot aside, it doesn’t matter where an illegal marriage (as defined in the USA) occurs – the USA has the right to honor or not laws of other countries. “Would the marriage be valid in Ontario?” may be a better question.