I was going to mention the "duration of the voyage" saw.
I dont know the details of how the Princess deal was cut and structured, but in order to be "legally" married in the 50 states (I don't know if it's a Federal law or not) you have to have a license from somewhere and, once the marriage is performed, it has to be registered with the governmental body which issued the license. Clear so far?
In our religion (Religious Society of Friends -- Quakers) we don't have ministers. People sort of "marry themselves" by rising during a Meeting for Worship and speaking to each other. There are, however, certain "legal" words that have to be included in the speakings. Every Monthly Meeting (sort of the body politik of the Quakers) has at least one Registered Celebrant who can sign the marriage license and return it to the State to certify that the "legal words" were spoken.
I don't think it's a legal requirement, but the exchange of speakings is in "plainspeak", that "thee" and "thou" stuff.
In theory (I only know this because I was tasked wth figuring it out for the Quaker "Vatican") a couple can get a marriage license, tuck a "registered celebrant" into their luggage and exchange their speakings in, say, Katmandu. They would be married in the state where the celebrant is registered and the license was issued, regardless of the geographic location.
Like everything else "legal", all this only becomes important when there is a legal wrangle, usually involving money.
We have friends who were married in China. Guess the spirit moved them. He's a law professor and former INS official. We got to talking one night about whether he and Helen were "really" married, since they got married in China but both were US citizens.
They went down to the local Court House and "redid" it some twenty years after the fact.