Jennifer Strange had taken part in the “Hold Your Wee for a Wii” game run by KDND 107.9 radio in Sacramento, which promised the winner a Nintendo Wii.
A work colleague said Ms Strange had reported her head was hurting hours after the contest and was going home.
Ms Strange, 28, was found dead on Friday at her house in Rancho Cordova.
Local assistant coroner Ed Smith said initial tests showed death was “consistent with water intoxication”.
Contestants were first given eight ounce (225 millilitre) bottles to drink every 15 minutes.
Contestant James Ybarra told Associated Press news agency: “They told us if you don’t feel like you can do this, don’t put your health at risk.”
Mr Ybarra said he quit after five bottles but remaining contestants started on larger bottles.
“She was telling me about her family and her three kids and how she was doing it for her kids,” Mr Ybarra said.
One of Ms Strange’s work colleagues, Laura Rios, said: “She said to one of our supervisors that she was on her way home and her head was hurting her real bad.
“She was crying and that was the last that anyone had heard from her.”
The station said its staff were stunned at the death.