In 1983 a British Airways S61 Helicopter on the regular scheduled flight from Penzance to St Marys (Isles of Scilly) crashed into the water on let down on a warm moist foggy/misty July Saturday morning.
23 were killed - 6 got out.
Haze limited their forward visibility so that they could not see the horizon, they were confident it was in excess of the VFR minimum of 200' ceiling.
Both pilots thought that the plane was still at 250 feet, though one of the passengers, a local, said that the cabin attendant had told her they were flying at around 100 feet shortly before the crash.