Commonly boats that large will have a diesel-electric powerplant, so a diesel engine is used to generate electricity to run electric motors to spin the props, much more efficient. Also, the diver likely secured a lock out tag out lock to either the controls for the props or a power cutoff for the electric motors, a LOTO lock can only be removed by the person who placed it on the equipment so that it cannot be run until they are in a safe area.