Taken from BBC boards helped me, may help others understanding the water colum.
I used a piece of paper to explain it to my self. Imagine you print out the screen image onto a sheet of paper. Now fold the paper in half along the line of the boat direction. now fold each side out 90 degrees where the bottom starts.
That will give you an upside down T. The vertical down leg is the water under the boat… the wings out to the side is the bottom going away from you out to the limits of your range setting. If you unfold it and lay it out flat you will see how the 3D is drawn in 2D.