On August 5, 1996, approximately 0925, Pacific daylight time, a Cessna U206F, N8313Q, operated by West Isle Air, Inc., and being flown by a commercial pilot, was substantially damaged when it overran the upwind end of a turf runway and slid over an embankment at a private airstrip on Crane Island, Washington. The pilot and passenger were uninjured. No flight plan had been filed, and visual meteorological conditions prevailed. The flight, which was an non-scheduled passenger flight from Anacortes to Crane Island, was to have been operated under 14CFR135 and departed Friday Harbor, an intermediate stop, approximately 0915. The pilot, who had not landed at this 1,800 foot, grass airstrip before, reported that prior to landing, he flew over the runway to check on its condition and to observe the winds. After making his observation, it appeared to him that the winds were calm, so he elected to approach the strip over the water, heading inland. After the touchdown, the pilot attempted to stop the aircraft on the wet grass runway, but in the last 100 to 200 feet of the remaining surface it became apparent that the aircraft would not stop prior to reaching the end. According to the pilot, the aircraft went off the embankment at the end of the runway while still at a speed of 10 to 12 m.p.h. After securing the passenger and aircraft, the pilot walked back up to the runway, where, according to the pilot, there was a tailwind that he estimated to be 8 to 10 knots. He said that even though the tailwind was present where he was standing, the wind sock was not indicating the presence of the wind.