You can't get heights or depths from a shaded relief raster. The purpose of shaded relief is described in this tutorial:
Shaded relief models indicate terrain displacements using a shadow
effect from evaluating the aspect and slope relative to the sun's
azimuth angle and altitude achieved with varying grey scale tones
resulting in the darkening of one side of terrain features, such as
hills and ridges (the darker the shading, the steeper the slope).
The tutorial is worth reading. There is also a dedicated web site about shaded relief http://www.shadedrelief.com/.
For getting the real elevation values you need a digital elevation model (DEM).