Surface contamination is deadly, particularly to bird/shore life, but dispersed oil is deadly throughout the water column (until it resurfaces, at which point it becomes deadly to birds again). There's relatively little life at the surface compared to the pelagic zone, so the impact of dispersants doesn't merely multiply the effect of the oil by the depth of the water, but actually moves that oil into the regions where life is densest.
Local fish production.
Number of words in public apology.
Average number of different bacteria cultures you can grow from a sample of water.
There's no end to ways you could measure incident response. Measuring based on visible oil makes the statement "the problem with oil spills is that you can see them".