The formula for the diffusion coefficient posted a few days ago showed that the rate of diffusion depended on the particle number density of the medium. This indicates that particles would diffuse faster in the air than in water since it is less dense. Evaporation probably played a significant role in the dispersion of the volatile organic compounds in the oil floating on the surface of the ocean. The molecules that are removed through evaporation can end up being adsorbed onto the surfaces of solids or broken down into simplier molecules through chemical decomposition via oxidation, thermal decomposition or photochemically through photolysis. The means whereby the molecules are broken down in seawater my involve similar processes as well as digestion by micro-organisms and other biological organisms.
The big question now is how much oil was processed by each mechanism and what is still present in and on the water and above the its surface. The fraction of the oil present in the water will probably have a different biological half-life for different layers of the ocean. A long half-life would help better distribute the effects of the oil spill over time as well as distributing it over a large volume perhaps reducing bioamplification by allowing more time for the elimination of toxic substances.
edit: A question that should be asked under the circumstances of the oil spill is, "How can we buy time for an ecosystem?" Another path that some of the oil followed was the burn off on the ocean surface.