Thanks for your answer!
But hypothetically speaking (or theoretically speaking, I'm not sure which one applies here better) if I could downsize a ship, a proper big ocean going ship, on which length would it stop being a ship and turn in to a boat? Like in the movie Downsizing. The downsized people were still people, even if they were very little people. And no, I haven't seen the movie. Just the trailer. Very intriguing.
If you make it small enough to fit onto another ship then yes it's a boat.