What is the Appropriate Hydrographics Level for Worlds with Sub-Surface Oceans?

I am trying to figure out the planet and trade codes for Ganymede which has a large, sub-surface ocean underneath an ice crust. At first, I was thinking that the hydrographics should be 9 or A, almost entirely water or waterworld, but then I was that hydrographics is supposed to measure surface liquid, which in this case would be 0. It just feels wrong to place its hydrographics at 0 from a trade perspective, but maybe I am thinking about it incorrectly. How would you spec Ganymede and similar worlds?