Unit 1 docked on the end of the dock. 1 occupant on board doing maintenance. Unit 2 trying to back up into slip next to unit 1. Boat was out of the position and driver of unit 2 throttled too much trying to correct and struck dock and starboard box of unit 1. Victim on unit 1 lost his balance and his hand was smashed between the dock/pole and gunwale of unit 2.