The depth of the ocean is ~250 meters in this spot, that's what's used. Since the coordinates aren't three dimensional Project-GC can not know if it's surface level or not. In case of the ocean it can be on the surface, or at the bottom. In case of land, it can be on the surface, in a high building, or deep below in a mine.
This is not an uncommon issue, every mystery cache in the ocean brings the same problem.