Hi Poltergeist,
There's certainly something very solid underwater in the area you describe. Whatever it is, I hit it at speed a couple of years ago and my centreboard shattered. It was an interesting sail home in a F5-6!
I wonder if the Stuka you referred to is the one that's still under the mud to the north of Dell Quay? The bomb was recovered in the late 70s (ish; can't remember the exact date) and taken out into the Solent and detonated. After the disposal team left I recovered a few very small parts of the aircraft that had been disturbed during the excavation.