Probably the most misused words in nautical lingo has to be the useage of the words "boat" and "ship." For example, almost all the crew on BC Ferries call their ships, boats; I'm forever saying, they aren't boats, they're ships.
I'm trying to remember what I was taught in the Canadian Navy
about this issue, and it seems to me (on shaky memory grounds here) that the difference between a boat and a ship had to do with number of continuous decks the vessel had. I seem to recall
that two or more continuous decks - covering the entire ship -constituted a ship.
I see looking at definitions of a ship on the net, that any vessel that had three or more (?) square rigged masts was a ship, not a helpful definition for today.
The remaining definitions are basically a large boat is a ship; again not very helpful. Even my Oxford definition wasn't very helpful.
Any other Navy
types here other than Canadian, were you provided a definition when you were or are active? Coast Guard definitions?