Re #548:
As Brian says, the muck on the water is algal bloom which manifests itself at this time of the year. It is not so bad in Dover where there are strong circulatory currents continuously washing it away but can look disgusting in Dunkerque West where it forms horrible great creamy clumps all over the surface. It is stirred up by the ships propellers and thrusters.
At the moment there is not that much of it around but it is present all over the Dover Strait in isolated striations, looking like soap suds on the surface.