A fisherman drew this to my attention, initially it was in very shallow water and I saw its wing tips above the water surface. I estimate about 1.3 m long and almost as broad.

The tip of the barbed tail seemed to be broken off.

When sitting on the bottom it looked like a grey granite rock (although there is no granite rock in the area). It was difficult to pick out, apart from when the tail was visible.

Added a still frame from a video showing close-up of the broken tail.

Added a short animated sequence showing it swimming.