RedactedNarrative: Ten subjects were swimming near vessel with motor still idling.  One subject returned to the vessel to eat some chips.  When jumping back into the water, his foot hit the throttle putting the vessel into gear.  The vessel then proceeded to make a small loop striking a sea wall and pier walkway.  No one was in the vessel at the time of the collision.  A resident of a nearby camp went to the vessel after hearing the collision and turned the vessel off.  The ten subjects were able to get on another vessel which brought them to the location of the accident and regained control of the vessel.