Mercury - Serpentine Belt Engine Drive System Rubber Composite 865615Q06. Overview. There is something reassuring about bringing your boat to life and hearing the engine settle into a smooth, steady rhythm. When everything is working together the way it should, it gives you a kind of quiet confidence-whether you're easing out of the no-wake zone or heading toward open water with a long day ahead. A reliable serpentine belt is one of those parts you rarely think about, yet you always feel its imp