Marine engines are under 100% load 90% of the time (because they're required to "push" so hard against the resistance of water). Compare this to car engines which are typically at 100% load only 10% of the time (only when accelerating hard or going up a steep hill). Therefore, marine engines are under far more stress and are more subject to wear. Microlon's engine treatments work by reducing friction inside the engine and help it to operate more efficiently.
Microlon Engine Treatment Instructions
See also Marine Engine Treatment Kits