My mind is cast back to an incident during a stop-over in Portugal while our squadron of ships was en route to the Mediterranean. The weather was perfect, the sea was calm and the beautiful sandy beach sloped gently into the water.
Four or five young officers ran into the sea until waist-deep and then dived headlong into the water. One of them surfaced face down and unable to move; luckily he was spotted quickly. He had broken his neck and had to be casevac'd back to the UK from Faro on a C-130 after an excruciating ambulance journey on rough roads. He was no older than 23 and, I believe, rendered a quadraplegic thereafter. No amount of risk assessment could have forecast or prevented such a tragedy. There wasn't even a hidden sand bar.