UNIT 1 WAS TRAVELING UPSTREAM ON LAKE SECESSION AT IDLE SPEED. UNIT 2 WAS ALSO TRAVELING UPSTREAM ON LAKE SECESSION AT APPROXIMATELY 45 MPH. UNIT 2 FAILED TO MAINTAIN PROPER LOOK OUT CAUSING THE PORTSIDE OF UNIT 2 TO COLLIDE WITH THE REAR STARBOARD SIDE OF UNIT 1. UNIT 2 PASSENGER WAS EJECTED AFTER CONTACT.