Might be part of a floating shipping container, but not likely at this scale.
Shipping containers typically float (if they float at all) just at the surface - very dangerous to things that can't tolerage a collision.
It sort of looks like a stitching error, but I'd expect the break to be noticeable by enhancing the waves around it. Still, I've noticed that some of Google Maps' stitching are done with varying levels of light level.. so this could still be part from an image where the water around the object looks pitch black, overlaid on top of other imagery of the water showing waves. The curves of the fading (which wouldn't make sense for a solid rectangular or flat object) still suggest a stitching/blending error.
No Longer There 
Shipping containers typically float (if they float at all) just at the surface - very dangerous to things that can't tolerage a collision.