Yes. They are vehicles built exclusively for the ocean floor.
Department of Motor Vehicles
bicycle,cart,boat
Department of Motor Vehicles
wheeled vehicles
The same laws apply to boats as apply to vehicles.
No, its not required by law, in most states. You'll have to check with your department of motor vehicles where you register the boat. You do have to have a title and registration for boat and trailer because of them being on the roads.
Car ans boat batteries will both work on either vehicles. A boat battery is just built a little tougher. This is to compensate for the rough banging and bumps of a boat ride. Boat batteries just cost a little more.
By walking...or using boat,steamer(if the stream is huge) and other water vehicles...
Henry Hudson used a boat and sometimes his own feet.
There are many insurance companies that cover aquatic vehicles. Three major insurance companies that handle boat insurance or quotes include Progressive, Allstate, and Geico.
Ferries (RoPax) normally carry both passengers and their vehicles.
If you're talking about vehicles, then you can have a Boat, a Minecart, and a Saddle. The Saddle lets you ride Pigs.