The pilot was demonstrating a confined area takeoff for a part 135 Visual Flight Rules (VFR) competency check from a remote lake in a float equipped airplane. The check airman, seated in the right seat, instructed the pilot to begin his takeoff run 90 degrees from the prevailing wind, and turn into the wind as the airplane accelerated for departure. During the takeoff water run the pilot failed to turn away from the shoreline. The airplane was almost on the step when the shoreline jutted out perpendicular to the takeoff run. He attempted to force the airplane into the air by applying flaps, and pulling aft on the control yoke, but the airplane impacted the shore, bounced into the air, and slid across the ground for approximately 100 yards before nosing down. The airplane sustained substantial damage to the left wing. The pilot reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation. In the recommendation section of the NTSB Accident/Incident Reporting Form 6120.1, the check airman stated that the accident may have been prevented if he had ensured the pilot understood his instructions, or if he would have discontinued the maneuver before an unsafe condition developed.