A float-equipped Cessna 208 aircraft (C-GIGO), operated by Seair Seaplanes Ltd., was preparing to conduct a flight from Ganges Harbour, BC to Vancouver Airport, BC with 1 crew member on board. Prior to the engine starting, the dockhand assisting the pilot untied the guide rope and held it in his hand. When the engine started the aircraft travelled forward pulling the rope from the dockhand. The aircraft began travelling forward until the propeller of C-GIGO struck the rear of a float-equipped DHC-3T (C-GUTW) operated by Harbour Air. After the initial prop strike, the pilot of C-GIGO quickly shut down the engine as the aircraft rotated 90 degrees striking C-GUTW with the right-hand wing. C-GUTW was tied to the dock with no one on board during the occurrence. There were no injuries; both aircraft sustained substantial damage.