Ships have sails to catch the wind and propel them through the water. Some modern vessels have sails out of nostalgia for the older sailing methods, while the actual historical vessels had no engines...
I think it has to do with the fact that water moves and so is never in the same place for very long. So, ... the next time a ship sails on that sea the water they sailed on the first time moved on...
not sure what you are asking, but the most commen Internet explanation is: it means making a relationship between two characters from a story, book or, movie. EX: a Harry Potter and Draco Malfoy...