It was American Tidal (2003 - 2007). It kicked off with a massive earthquake that hit San Diego. A huge portion of it sunk into the ocean and was subsequently dubbed "Sub Diego." Initially, they thought everyone drowned. But when they went to recover the bodies, it was revealed that thousands were able to breathe underwater.
Like the current storyline in Invaders, it involved a chemical being used to mutate humans into water-breathers (a scientist developed it for the purpose of allowing people to survive if the polar caps were devastated by global warming.) It was never meant to be used. But when the earthquake hit, the facility plunged into the sea and the chemical was mixed into the saltwater where it rapidly mutated.