Well, I can think of two reasons why not:
1: Because, being on a boat would make them instantly want to break into song, singing The Lonely Islands I'm On A Boat. Because, they would be on a boat, and that's what all sane people want to do, having heard that song.
2: Because of very hazy legal ramifications and uncertainty. There seems to be no guarantee that it would be legally recognized. And as a previous poster mentioned that ships still operate under the laws of the country whose flag they sail under, it may be illegal.