It looks like a whale making a huge splash in the shallow water just off the southern coast of Brownsea Island, Poole Harbour, England. I really don't know what it is...no trace of it exists in the BEV's or GM.
No Longer There, Unexplained
By: daggerwell
I suspect this is one of those rare maps that legitimately belong in the "unexplained" category rather than "info wanted."