Oil in a normal spill contains heavier fractions that cling to the sea floor, as well as the lighter fractions that rise to the surface. The BP spill was not the first one in history to cover some of the sea floor. They all do that.
If the oil spill (either natural or 'Corexitised') had been large enough to cover the entire Gulf floor in a layer an inch thick then we could reasonably expect some impact on spawning. But one molecule of oil for every ~ 400 sq. miles of sea floor has simply negligible impact.
We all know that normal oil spills contain heavier elements. I am not disputing this.
One of the features of Corexia is to take the lighter fractions, that would normally cling to the surface (as you point out) and PUSH them to the ocean floor. Moreover, the amounts of Corexita were in excess of any amounts previously used AND in excess of the amounts authorized by the EPA. This administration used the Gulf as guinea pigs.
From what I understand, it is the enormous blast of Corexita that is causing the sickening human health problems in the Gulf. But I am going to wait for more solid information.