It's part of the ocean floor. I think the captain did a little impromptu dredging on his pass-by of the island, to impress the girlfriend who was never on the bridge.....
Sorry, I should have posted the credit for the photo.
I got a job application from a guy a few days ago, someone looking to captian our submersible vessel on the 2014-15 series of Mediterranean Sea missions. I better have a look at the name... and the references!