Abstract: Pontoons along both sides of the vehicle can be filled with either air or water, at the user's preference. When the pontoons are filled with air the vehicle floats in water and is driven along the water surface by a propeller and also by a tread drive. When the pontoons are filled with water the vehicle sinks in water and is driven along the bottom by, again, the propeller and treads. In either event when the vehicle comes to a dry surface it continues to be driven along that surface by the treads. The pontoons are provided with a hole and plug for use in filling or draining them. Preferably the interior spaces of the pontoons communicate through a cross-connection chamber that rougly equalizes the amount of water in the two pontoons, so that the vehicle has minimal tendency to capsize or ride in the water at a cant. An unusual two-worm-and-worm-gear drive train provides power to the tread drivers both fore and aft with high mechanical advantage and very few moving parts.