As an experienced Maritime Pilot, I had many hours flying at 100 feet over the sea, but with the benefit of a Radar Altimeter.
Then I became a QFI on Jet Provosts at Linton. One of the other QFIs had done a tour at Finningley, flying Nav students. He was the most experienced low level JP pilot at Linton.
One morning he was flying solo on Staff Continuation Training. The weather was hazy, calm wind and the reservoirs in the low flying area were like glass. I would not go near them.
He did, flew too low (authorised to 250 feet minimum) and in a turn dug a wingtip into the water and died. QED.