A cone of depression occurs solely in locations where there are wells, so they'll only be found on land. Normally the water table, the constant water level underground, sits at a certain depth below the surface.
When a well is activated, suction occurs at the center around the well and the ground water moves towards it instead of the normal discharge area. Think of what happens when you empty a sink of water or drink through a straw. The area below the original water table that is now void of water is called the cone of depression. On a diagram it is a very noticeable cone-shaped formation.