When I was a kid, I got cut off by the sea coming in behind the beach area I was on. I was with my parents (along with many other people). We had been at the waters edge and had been aware that the tide had turned but didn't realise that the shallow undulations of the beach behind us had filled with water and had a strong current.

We were alerted to the danger by a chap on horseback who was charging up and down the beach calling for people to head back towards the shore.

I don't know if the chap on horseback was a "lifeguard" or from the local authority but it certainly prevented a more difficult return to shore which for me meant returning through water up to my neck with sand under my feet that "gave way" if you stopped walking (it was the current undermining the sand).

I'm guessing that the Camber Sands beach had similar "shallow undulations" that, when the tide was out, didn't seem like anything untoward or dangerous.