There's more to this than meets the eye ...

UPDATED 6/8
Via the NY Times:Plumes of Oil Deep in Gulf Are Spreading Far, Tests Find

The government and university researchers confirmed Tuesday that plumes of dispersed oil were spreading far below the ocean surface from the leaking well in the Gulf of Mexico, raising fresh concern about the potential impact of the spill on sea life.

+++End of Update

Despite repeated denials by BP and NOAA, the evidence for the presence of massive submarine plumes of spilled oil is growing. While we would expect the oil to be floating on the surface, a couple of factors can help keep it submerged.

Firstly, crude oil varies in density. Petroleum engineers use a measure called the API Gravity which is expressed as "Degrees API"(º API) and calculated by:

Where:
A = API Gravity (º API)
s = specific gravity of the oil at 15.6º C. (g/ml)

For example, a light fuel like gasoline with a specific gravity of ~0.74 has an API gravity of ~60º API. The deepwater crude oils in the Gulf have API gravity ranges from 15º to 60º API, with most crudes falling between 25º and 35º API, significantly heavier than gasoline.

The second contributing factor is drop size. The rate at which oil ascends in the water column is given by the following equation using Stokes law:

Where:
V = velocity of rise (cm sec-¹)
D = radius of the oil drop (cm)
g = acceleration due to gravity (cm sec-²)
dw = water density (g cm-³)
do = oil density (g cm -³)
µ = viscosity of water (dyne sec cm-²)

From this equation, we can see that the (dw -do) term will get decrease as the density of the crude oil increases, decreasing velocity. Since velocity also varies with the square of the drop size, smaller drops will rise slower than large ones.

This is why BP is releasing dispersants at the well head. The dispersants break the oil into smaller drops, slowing the rate of rise and allowing more oil to be advected out of the immediate area by submarine currents before it reaches the surface.

Via Tampa Bay Online [TBO.com]
USF scientists to announce source of underwater oil pools

TAMPA - University of South Florida marine scientists today completed testing on two large underwater pools of oil some 35 miles northeast of the busted Deepwater Horizon oil rig in the Gulf of Mexico and are expected to announce tomorrow, with scientific certainty, whether or not the greasy pools come from the ruptured well.