Submarine

A rarepair/unpopular/unknown ship, not known or liked by many members of the fandom. You can't sink what's already sunk.

"It's annoying, I love them together but it's such a submarine that no one else even thinks of it."

Ghost Ship

A ship in which both characters (or all) involved in the ship die in canon. It may be sunk but it's still sailing.

"I watched a movie with a really nice ship in it, unfortunately, it was a ghost ship by the end of the movie"