Explain why the ocean floor is below sea level (depressed) while the the land is above sea level (elevated)?

Question

(Hint: think in terms of the nature of the continental crust and the ocean crust and their density differences)

Answer 1

???

Because water is heavier than air?!?!

Answer 2

The Earth's surface is dictated by the placement of it's many tectonic plates, all of which are "floating" on the upper mantle.  These plates are composed of continental and oceanic crust.  

Oceanic crust is basaltic in nature, with a typical density of 3.3 grams per cubic cm, and is most often under 10km in thickness.

Comparatively, continental crust which is granitic in nature, is less dense, at only 2.7 grams per cubic cm.  This difference in density in conjunction with it's considerably greater thickness (35 to 40 km in most places) causes continental crust to float comparatively higher, leaving oceanic crust as an ideal basin in which surface water may collect.

I'm sure I've made a few oversights, but that's how I've come to understand it...

How did I do?