I just had a thought, maybe SE can show how far above the 'sea level' of the planet as it would be interesting to see how high the mountains are, or just out of curiosity in general.
For planets without water, it could either calculate from the lowest elevation or count from the planets median or average altitude. The second method would work better for planets with lots of deep canyons or for small objects.
I know Space Engineer is going to make it so that in the next update, it'll count your height above ground from the water surface and underwater it'll show as negative, so something like this could be a next step.
Problem: How much water before a planet has a sea? Is a small pond sufficient? What if there are two seas at different altitudes? Relative to median level regardless of whether seas are present could work as a general definition, though.