The oil spill is having a big impact on the gulf seafood industry because as the oil moves inland, the size of safe fishing areas is reducing and some fishing waters have been closed. The US Food and Drug Administration (FDA) and the National Oceanic and Atmospheric Administration (NOAA) work to monitor the oil spill situation and will continue to close waters that become contaminated and open waters when they are eventually clean. No seafood can be harvested from the closed fishing waters. You can take a look at this map maintained by NOAA to see which areas are still open to fishing.
In addition, according to the FDA, the dispersants (chemicals that cause the oil to disperse into the water rather than float on top of the water) used against the oil spill aren't a problem either, because they don't appear to accumulate in seafood. So the gulf coast seafood you find in your grocery stores and restaurants is safe to eat, and continued monitoring of coastal waters should prevent any contaminated seafood from reaching your table.
US Food and Drug Administration. "Gulf of Mexico Oil Spill Update." Accessed July 21, 2010. http://www.fda.gov/Food/ucm210970.htm#safety.


