What Is The Definition Of A Boat?

When people think of a boat, they usually think of a boat that is floating, however it is still a boat even if it's not a floating. It's still a boat if it's sinking or if it's sunk. A boat can be made from fiberglass, which nowadays is the most common material for recreational boats, wood, steel, aluminum, rubber, PVC, hypalon or just about any other material that one could think of. I've even heard of boats being made out of plastic drink containers. 
The division between boat and ship is not precise, although The Navigational Rules make a distinction at 20 meters, or about 60 feet.  Craft and small craft usually carry the same meaning as a boat.  The term vessel is used particularly in legal and regulatory contexts without reference to size.  Yacht deserves special consideration.  It often connotes a sail or powerboat over 40 feet in length with luxurious accommodations.  It is also widely used when referring to prestigious government craft such as a presidential or state governor's yacht.  In sailboat racing, every competing boat is a "yacht" without regard to size or accommodation.
A boats primary means of propulsion places in either the powerboat or sailboat category.  Sailboats are propelled by wind and sails.  Most sailboats are over roughly 18 feet long have inboard gasoline or diesel auxiliary engines; smaller ones often have an outboard motor. Small, open sailboats are called day sailers and those used primarily for racing may be known as racing dinghies, although the word dinghy is usually reserved for a very small boat used to ferry crew and supplies. 
A boat propelled by machinery is known as a power driven vessel. This includes sailboats using auxiliary engines, whether or not they have sails up. A motorboat includes craft propelled by gasoline or diesel engines. Some motorboats are now we'll propelled by environmentally friendly electric motors and storage batteries. Some boats are now powered by solar power, partly or completely. 
Other types of boats are; motorsailers, cruisers, houseboats, hydroplane, hydrofoil, utility boats, rigid hull inflatable boats, sailboards, surfboards, and I could go on and on. There is a boat out there for anyone.