As far as I’m concerned, ships are ships, and boats are boats, and never the twain shall meet. Just don’t ask me to define the exact demarcation point…
There is no precise distinction between a ship and a boat. It’s more a matter of custom and usage…or plain common sense. After all, like a lot of things in life, size matters, but in the case of ships vs. boats, size alone can be confusing.
Consider submarines: traditionally referred to as ‘boats’ (never mind the fact that their official names always includes the prefix ‘USS’ - denoting United States Ship.
Here’s another weird rule of thumb: ships have to be big enough to carry boats, and boats have to be small enough to be carried by ships. But there are many exceptions. All of the following could just as easily be called ships. PT Boats, Ferry Boats, Ocean-going Tugboats, Tow Boats, River Boats, Fire Boats, and Commercial Fishing Boats…
As for the measuring tape method - I prefer the USCG internal designation for ships. In the USCG everything 65 feet and over gets the designation of a ship anything less is a boat. A USCG Cutter 65 feet or greater in length, with living accommodations for the crew, carries the ship prefix USCGC. Anything less, is a boat.