A boat is a watercraft of any size designed to float or plane, to work or travel on water. Boat is also used to describe space craft. In both Fleet and "wet navy" terms, a boat is a vessel small enough to be carried aboard another vessel (a ship).
Some definitions do not make a distinction in size. For reasons of tradition, submarines are usually referred to as 'boats' rather than 'ships', regardless of their size and shape. Also fighter craft are not usually referred to as boats.
Boats have a wide variety of shapes and sizes and construction methods due to their intended purpose, available materials or local traditions.