Yes, you are right major difference in ship and boat is size when a boat exceeds the size of 15 to 20 metre in width it normally called as ship.
When Does A "Boat" Become A "Ship" ?
A boat becomes a ship, when you put a boat on it.
Some boats never become ships! Like some boys never grow up! :-)
When a size of a boat became 10 to 20 times larger.