RedactedNarrative: A vessel (1966 15' Larson All-American) with 3 occupants was traveling at approximately 5-10 mph when the motor struck an unseen underwater object (described by the operator as more likely wood than rock) in an estimated 6-8' of water. The motor (80hp Mercury outboard) popped up then slammed back down against the transom causing a crack in the transom. Vessel began to take on water then soon sank in 4-5' of water. Occupants pushed the vessel to a shallower depth near shore then reached land without injury.