Dispersants are used in an oil spill to break up the oil and help in its reduction through natural processes. The dispersant is basically a surfacant which makes the oil more soluble in water by creating an emulsion consisting of tiny droplets of oil in water. Dispersants rated by the hydrophilic-lipophilic balance. In nature one often finds proteins whose function are to mix substances which tend to be immiscible. For example milk is an emulsion in which oils and proteins are held in suspension as miscelles which are microscopic and can be smaller than a cell. The white color of milk is due to the fact that its particles scatter all frequencies of light equally. The particles are what makes milk "milky" and allow it to be thinned by adding water. This is what those trying mitigate the oil spill do with the oil. As a result it ends up in the water column.
The dispersant initially used by BP was Corexit 9527 which is slightly toxic and the EPA has asked BP to switch to the less toxic Corexit 9500. Another dispersant used in oil spills is Dispersit. Before BP can do anything in the Gulf it has to file a plan with the government which needs to be approved. BP has posted its plans for the use of dispersants as part of its response to the oil spill on its website. These plans included the subsea injections of dispersants during the initial capping operation and topkill. The intent was to let nature take its course. They also note the need to monitor the water column.
Oil can also be broken up by mechanical means as was discovered by Joseph Plateau about 140 years ago. He altered the physical properties of oil in water and rotated the oil breaking it up into smaller particles. Physical forces act on the particles of oil and allow it to disperse through diffusion which is driven by the random motions of the particles like that in Brownian motion. One would expect the oil in suspension in the water column to spread both vertically and horizontally through diffusion and also by convection and currents.
There are still concerns about the persistence of the oil in the water column and its subsequent effect on the environment.