Two questions for you experts

[goldorak]

Originally Posted by Pisces

I've heard that rumour also. But when you take a good look at seabed features you'll notice the darkness is allways on the same side of it (lower side iirc), no matter where on the world it's at. So it must be some kind of gradient/slope thing like Dr Sid said. As accoustics it makes no sense.

Exactly, light blue = shallow water, deep blue = deep water.
Nothing more, nothing less.
The colors don't represent sonar performance per se, but if you consider the underwater mountains and valleys then yes they give topographical information on where to hide for maximum effect and where to listen to achieve maximum performance.