Mercury - Serpentine Belt Engine Drive System Rubber Composite 865615Q05. Overview. Some parts on a boat may be out of sight, but you feel their presence in the way the whole system behaves. A serpentine belt is one of those quiet workhorses. When it's right, the engine feels balanced and willing, the accessories hum along without strain, and the day on the water just unfolds smoothly. For tow sports boats and MIE engines, this Mercury Marine part brings that calm confidence back into the equati