Treading water as your boat moves away from you with nobody else on board, is the stuff of sailing nightmares – and the plot of a Hollywood film – Open Water.

But for the second time in a month, sailors have been rescued after treading water for hours.

On 18 July, Hamble yacht broker, Prue Nash, was rescued after two hours afloat in the Irish Sea.

But when four University-age Americans decided to take a midnight dip off a 31ft yacht in Lake Michigan last weekend, it proved fatal for one – with another still missing.

News reports describe how all four, two men and two women, left the engine running and the sails furled as they swam off their yacht, Joan’s Arc.

The yacht drifted away from them and the four became separated three miles offshore. None were wearing lifejackets. 

Five hours later, the two 19-year-old girls were rescued by a fisherman who told reporters that the air temperature was 70°F (21°C) – a factor that helped the girls survive.

A police search discovered the body of 21-year-old James Shepherd, who was an experienced sailor. Another man is still missing.