The entertainment news outlet TMZ said a male actor dived from a 30-foot platform into a water tank but did not immediately resurface.
Panicked colleagues pulled him out and performed CPR before he was rushed to hospital, where he is still being treated, the website reported.
“It was a full house. It happened in front of everyone,” spectator Daisy Anguiano told a local ABC affiliate.
He appeared to be unconscious while being “dragged from the water,” she said.
“WaterWorld” is a special effects-filled live show based on the 1995 post-apocalyptic action film starring Kevin Costner.
Although the film itself was seen as an expensive flop, “WaterWorld: A Live Sea War Spectacular” opened at the Los Angeles theme park the same year and has remained an enduringly popular attraction ever since.
Lasting around 15 minutes, and performed several times each day, the show sees performers interact with fans, perform hair-raising stunts on jet skis and high-wires, and trigger explosions using movie-set technology.
Around 170,000 pyrotechnic effects are fired per year at WaterWorld, including giant fireballs rising 50 feet (15 meters) in the air, according to the theme park’s website.