As far as i know, the earth's crust below and around the impact gets displaced by a fair amount. I think the crust acts like a rubber sheet bouncing backwards and forwards. This most likely contributes toward the displacement of water above.
That reminds me, I remember reading some NASA stuff my dad would bring home talking about simulations of impacts. While there are very few on earth there are vast numbers on the moon and other planets.
I think the forces and energies are so large that the fluid dynamics dominate.
