I am pretty sure it is an abandoned section of Mulberry Harbor from WWII. The wiki on the Mulberry Harbour states, "...a section of it remains embedded in the sand in the Thames Estuary, accessible at low tide, about 1 km off the coast of the military base at Shoeburyness."
http://en.wikipedia.org/wiki/Mulberry_harbour
We have a couple other examples in VGT:
http://virtualglobetrotting.com/map/mulberry-harbor/
http://virtualglobetrotting.com/map/segments-of-mulberry-harbor-1/