The Boat is a dinghy boat in Grand Theft Auto 1, it can only be seen in two missions. The Boat, unfortunately, is not meant to be driven by the player as trying to jump on it will cause the player to plunge into the water and drown.
It can be driven with mods, as you can switch its number with any other vehicle (its number is 13), and any vehicle of that type will be replaced by it. It is very slow, slightly faster then the Love Wagon, but less agile. Strangely, it can also be driven on land, as the game isn't advanced enough to know that a boat should become 'grounded' on land. It is relatively easy to mod it into the PC version of the game.