Another difference between Ixtoc is the area hit. South Texas beaches versus the marshes.And booms were effective. Some oil escaped around, but Impacts to the estuaries were minor.
Oh, I suspect some oil also made its way ashore around Ixtoc. The one thing I have not yet located is a science study of the immediate vicinity of the Ixtoc well itself.
But such documentation "does" exist for the Persian Gulf "Saddam Hussein" spill. The worst affects were seen in the salt marshes, which took up to ten years to recover....BUT...the magnitude of the oiling was hugely greater than Ixtoc was or BP is likely to be. Beaches were completely covered up to the high-tide water marks with al layer of oil, and oil seeped down into the soil to a depth of 60 cm.
In areas where the oil coverage was not total, recovery was a lot faster.
http://www.c-3.org.uk/Multimedia/Reports/Gulf%20war_Poonian.pdf