If the diver is 19 meters below the surface, we can represent the depth as a one-dimension measure, where the surface has a value of 0.
Then, if he is 19 below the surface, he is at x=-19.
He has to travel from x=-19 to the surface (x=0), then the difference is:
Surface - Actual Position = 0 - (-19) = 19
He need to travel 19 meters towards the surface.