0 represents sea level.
This elevation is relevant to both positions that the scuba diver is at because, when they are on the boat, they are on an elevation of 5 feet above 0 - or sea level.
When the scuba diver is 38 feet under however, it shows that they are obviously under sea level or -38 feet from the elevation line of 0.
the elevation of 0 is relevant because positioning can be calculated - like how, for the scuba diver to get from point A (5 feet above) to B (38 feet under) they would have jumped down 5 feet (to reach sea level) and then another 38 feet - resulting in a total elevation jump of 43 feet.
Hope this helps a little.
