For the last few days, attention has understandably been directed at the shores of the Gulf Coast as oil has started to wash up on beaches and in marshes. But last week I had the chance to see the effects of the spill from another perspective - when I dived into the oil slick a few miles off the Pass a Loutre wetlands in southern Louisiana. What I witnessed was a surreal, sickening scene beyond anything I could have imagined.

As the boat entered the slick, I had to cover my nose to block the fumes. There were patches of oil on the gulf's surface. In some places, the oil has mixed with an orange-brown pudding-like material, some of the 700,000 gallons of a chemical dispersant called Corexit 9500 that BP has sprayed on the spreading oil. Near Rig No. 313, technically a restricted zone, the boat stopped and I (wearing a wetsuit, with Vaseline covering exposed skin) jumped in.

Only a few meters down, the nutrient-rich water became murky, but it was possible to make out tiny wisps of phytoplankton, zooplankton and shrimp enveloped in dark oily droplets. These are essential food sources for fish like the herring I could see feeding with gaping mouths on the oil and dispersant. Dispersants break up the oil into smaller pieces that then sink in the water, forming poisonous droplets - which fish can easily mistake for food.

Though all dispersants are potentially dangerous when applied in such volumes, Corexit is particularly toxic. It contains petroleum solvents and a chemical that, when ingested, ruptures red blood cells and causes internal bleeding. It is also bioaccumulative, meaning its concentration intensifies as it moves up the food chain.

The timing for exposure to these chemicals could not be worse. Herring and other small fish hatch in the spring, and the larvae are especially vulnerable. As they die, disaster looms for the larger predator fish, as well as dolphins and whales.

As I swam back to the surface, some big fish came up to the boat - cobia, amberjacks weighing up to 60 pounds - looking for a handout. These are the fish that have made the Gulf a famously productive fishing area. But they rely on the forage fish that are now being devastated by the combined effects of oil and chemical dispersants. In a short time, the predator fish will either starve or sicken and die from eating highly contaminated forage fish.

Yes, the dispersants have made for cleaner beaches. But they're not worth the destruction they cause at sea, far out of sight. It would be better to halt their use and just siphon and skim as much of the oil off the surface as we can. The Deepwater Horizon spill has done enough damage, without our adding to it.

Susan D. Shaw is a marine toxicologist and the director of the Marine Environmental Research Institute, a nonprofit scientific research and educational organization.