The Stronghold was stated as being 50km in diameter.
Using that as a measuring stick the solid structure is somewhere between 10-15km tall and the floating obelisks somewhere between 15-20km tall. Which means to fully cover it to the point of invisibility like we saw we are talking about a water depth of somewhere between 30km to 40km.
From what we were told the Ba'ul were, at most, maybe eighty to a hundred years behind the Federation.
Of course from what we saw, the Ba'ul were actually more advanced then the Federation in certain area's.