A concrete barge which should have been used as part of the floating roadway of the Mulberry harbour for the D Day landings. According to locals, this section came adrift before it could be towed to the Normandy coastline on the morning of 6th June 1944.
http://maps.live.com/default.aspx?v=2&FORM=LMLTCP&cp=sft17cgy5f4y&style=b&lvl=2&tilt=-90&dir=0&alt=-1000&scene=10946426&phx=0&phy=0&phscl=1&encType=1
Here's what a Mulberry looks like above water:
http://virtualglobetrotting.com/map/13880/
http://virtualglobetrotting.com/map/25475/
http://www.citiesofscience.co.uk/go/London/ContentPlace_2677.html