On June 19, 2002, about 1700 Alaska daylight time, a float-equipped deHavilland DHC-6-300 airplane, N353PM, sustained substantial damage while water taxiing at Ketchikan, Alaska, when it struck another float-equipped deHavilland DHC-6-300, N235SA, which also sustained substantial damage. N235SA was moored to the dock when the accident occurred. Both airplanes were being operated as visual flight rules (VFR) local area sightseeing flights under Title 14, CFR Part 135, by PROMECH INC., dba, Seaborne Seaplane Adventures, of Ketchikan. The captain, the first officer, and the 15 passengers on N353PM, were not injured. The captain, the first officer, and the 12 passengers on N235SA, were not injured. Visual meteorological conditions prevailed, and VFR company flight following procedures were in effect. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on June 20, 2002, the captain of N353PM said he was preparing the airplane for departure when the accident occurred. The airplane doors were closed, passengers seated, and engines running at idle power. The airplane was still tied to the dock, and the shore power cable was attached. The captain said he gave the appropriate hand signal to the dockhand to disconnect the shore power cable. He said after disconnecting the shore power cable, the dockhand released the mooring rope holding the airplane to the dock. The captain said an additional hand signal is used to signal the dockhand to release the mooring line, and that signal was not given. The airplane, N353PM, surged forward under idle power, and its nose collided with the tail of N235SA, which was moored to the dock about four feet in front of N353PM. The captain of N353PM said he was able to shut down the engines before the two airplanes made contact, but the propellers were still wind-milling during the collision. N353PM sustained substantial damage to its propeller blades and nose. N235SA sustained substantial damage to its elevators and vertical stabilizer. During a conversation with the IIC, the director of operations for the operator said the captain and first officer of N235SA had just finished loading passengers, and were standing on the dock at the time of the accident. N235SA was moored to the dock, and the engines were not running. He said, after the accident, the floatplane operation has been moved to another dock where the airplanes are separated by at least 50 feet. He said the operations manual/procedures have been revised, and dockhands no longer handle departing airplanes. Under the new procedure, the captain (PIC) is at the airplane's controls, and the first officer releases the mooring lines.