DefenseTech.org (I read it for the SF story ideas) has an article about newly released records from Russia about close encounters with alien spacecraft – underwater!
I seem to remember a movie about an alien spacecraft on the bottom of the ocean, but I can’t think of any alien submarine stories. It would make sense that an alien civilization might be water dwelling and come to our planet and pretty much ignore the dry land.
It goes into the story idea list, but I am not sure what kind of story I would write. It would make a good anthology because there are so many possibilities.
It would make a good screenplay. I have to think about this.