Here's an old tub, moored at Maldon in Essex.
Now I think this is a ship because a captain once corrected me when I called his ship (the biggest container vessel I'd ever seen) a "nice boat". "You mean nice ship!, ships", he said, "carry
boats" Never made that mistake again!
This vessel is carrying a boat, ergo, it's a ship! (I'm not sure that this rule of thumb applies to all vessels though?).