No idea about the authenticity of the story, but...
(1) There was a satellite factory, building Sunderlands, on the shore of Windermere during the war.
(2) It's very deep.
(3) I think at-least one Sunderland was damaged on the lake during the war or shortly afterwards.
Presumably there's a local history society up there somewhere who may have published something on the history of the Windermere factory - anybody got anything? There is a chapter in the book "Corresairreville" (Sp?).
Having said that, the picture looks thoroughly bogus. No engines, little damage, no apparent widescale damage (after 60 odd years) and nothing else on the bottom. Looks a trifle dodgy to me.
G