One should consider that the colour of the sea is depending very much on light condition, what is inside the sea (algae), how shallow or deep it is or if it is real clear water or if there is much plankton or dirt in it and water temperature and the ground composition. I guess also the air may alter the appearance of water.

I remember having visited an Island just short south of Brittany in France (Atlantic ocean) so quite north and nothing that should look too different from the Channel.

Well, it did. It looked on this particular day as if I had been in the Carribean with locally turquoise cristal clear waters. The same water may look completely different on a rainy day.

Some images of the Blautopf, a location in Southern Germany that translates roughly into Blue Pot called this way for some good reason as you immediately will see in the pictures. As I have seen it with my own eyes: it is definitely and really very blue:

http://en.wikipedia.org/wiki/Blautopf