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Shoeburyness: Tidal paddling pool

I am speculating that this rectangular structure is a tidal paddling pool. These fill up with seawater at high tide. Then, so the theory goes, when the tide goes down, the sun quickly heats up the retained shallow water making it an ideal pool for children. These man-made tidal pools are usually swimming pools, but this one is too shallow and small.

The red blob in the distance is a windsurfer making their way to the mudflats off Shoebury Ness, while on the horizon are two anchored ships in the Thames Estuary.