Abstract: An amphibious vehicle has a heat engine with an ambient air cooling system primarily employed for cooling the engine during travel on land, and an external water cooling system primarily for cooling the engine during travel on water. The engine is mounted in a sealed interior of the hull body of the vehicle. There is a retractable front steerable wheel and at least one retractable rear wheel. A propulsion unit, preferably a water jet pump, propels the vehicle in water. The engine is directly cooled by a re-circulating liquid coolant, and the cooling systems include a heat exchanger having an air chamber through which ambient air flows, a liquid coolant chamber through which the liquid coolant flows, and one or more fins exposed outside the hull body in contact with external water during water travel. In a preferred form the vehicle is a personal watercraft in water and a three-wheel motorcycle on land.