Underwater dispersants attach to the oil before it can reach the surface, thus minimizing the amount of oil that eventually lands on shore.
Underwater dispersants break up oil by bonding to the oil molecules and forcing them to separate from regular sea water below the surface. This renders large underwater oil slicks into smaller, more quickly degrading oil droplets.
Underwater dispersants are not without their own drawbacks. Though less toxic than the oil spill it’s meant to address, the chemical itself is actually toxic and could potentially cause damage to underwater ecosystems.