Imo, the mountain ranges were pushed up by the flood waters which sank the oceans to their present depths, the pre-flood earth being relatively smooth surfaced.
So six miled of water with a density of 1 kg/L is going to push 20 - 3 = 17 miles of rock with a density of 3 kg/L upwards. Try that in your kitchen, and get back to us on how much uplift you get. And keep in mind that you are starting with rock, not air, in your seabeds-to-be.