RedactedNarrative: Operator of V1 stated he hit V2 that was docked in a private boat slip. V1 Operator was pulling his grandchild on a tube while making a hard left turn; he looked back at the tuber while continuing to turn and when he looked forward  he saw he was not going to be able to make the turn in time to miss the docked boat (V2).  He pulled back on the throttle but could not stop in time.  V1 struck V2 in the stern causing damage to the ladder and motor of V2 and damage to V1's starboard bow.