During the 1800's, when couples were married "at sea", where would their marriage be ?
Would it be their country of origin, the country they are immigrating to, the country they are sailing by, or anywhere at all?

What an interesting question. Found out through a girlfriend that modern weddings on ships can't be performed in international waters - they are generally done while the ship is in port.

In the 1800s - my best guess would be that the marriage would be associated to the country the ship is registered with.