there's a few different theories on this. Some have said it's due to the temperature of the object matching the temperature of the ocean behind it, and therefore it blends in with the background giving the appearance of going into the water. I think it's a little more complex than that -it's a combination of the grey infra red image of the object not contrasting with the grey ocean, but also the video compression of a bland grey scene that causes macro-blocking artefacts and therefore a loss of detail.
Basically - it's an infra red camera effect.
