Boxer has nothing to do with air-cooling. Many boxer engines are water cooled including the current BMW engine and Subaru engines. Boxer simply refers to the horizontally opposed cylinder configuration.A Boxer engine is a horizontally opposed air cooled engine. In the case of BMW bikes, it is a twin but larger numbers are common as per Porche cars. The K engine is largely a four cylinder in line water cooled car engine laid on its side. As different from each other as is possible to be!