Rotorbrake is never applied during a ditching as this will cause the helicopter to yaw making the whole situation worse. You wait until the blades stop turning and sometimes you're even lucky enough and the helicopter will stay upright. The egress procedure we get taught in the military is to wait until all violent motion stops before exiting the aircraft to avoid getting hit by the turning blades on your way out. We have to go through lots of training to be able to execute an egress successfully. And I can say from experience that being upside down strapped into your seat underwater is not the most comfortable position to be in. For an untrained person this doesn't end well most of the time.