meaning of water colors on map
 

There are color gradients on the charts that tell you the depth of the water in that area, from dark blue to white. I didn't see any key as to what the depths represented where, so I did some test dives to find out.

dark blue: >500 feet

med. blue: >300 feet

light blue: >200 feet

sky blue: >100 feet

white: <100 feet

So in the darkest blue, feel free to dive to test depth. Med. blue, you may not be able to dive as deep as you want to. Light blue, deep diving to avoid depth charges is out, if attacked try to reach deeper water ASAP. Sky blue, danger zone. Find deeper water immediately or you risk destruction. White, only submerge here in the most extreme circumstances, and don't go below periscope depth.

Now, you could use active sonar to measure the depth...what? that cool feature (which we had in SH3) is no longer in the game? Gee, poor us.

hope this is of some use :D

It's in the game. Depth to Keel order is under the wheel button on the far left of the command menu. It has a keel symbol with a double ended arrow underneath. ;)

That's still in the game. It's on the first toolbar, the one marked with a steering wheel.