Let's say BP spills 5 million liters of oil. Since a cubic meter is 1000 liters, this is 5000 cubic meters.
That is a lot, but the Gulf is very large:
A square mile is 2.6*10^6 square meters.
So the gulf is about about 1.6*10^11 square meters by 2000 meters or about 3.2*10^14 cubic meters.
So the ratio of oil volume to gulf volume is about 1/31,720,000.
If the my bathtub holds 20 liters, I would need to add about 1/1586 of a milliliter of oil to resemble the total Deepwater Horizon spill. I doubt I'd notice it.
Of course the local effects of the oil are very large. I'm also pretty sure I've made some arithmetic errors (it's quite late and, alas, I rarely do even arithmetic these days). Still, the gulf is really vast.