Briles also jumped in the water, but the boy had already been rescued.
Amormino said it started when Briles and his girlfriend got into a verbal altercation aboard the boat. Briles' two sons from a previous marriage, a 7-year-old and 6-year-old, were with him on the trip. Both boys apparently witnessed the argument and the 7-year-old was visibly upset, Amormino said.
Witnesses said that's when Briles picked him up and tossed him into the water.
On Tuesday, Briles told KTLA that he did nothing wrong and "we were having fun, it was a harbor cruise!" Briles said he did not hit his son and never has, adding "I did nothing except jump in the water with my kid."
Deputies who took Briles into custody Sunday said he was combative and resisted arrest, and they believe he had been drinking.
When asked if he would have done the same thing had he not been drinking, Briles said, "Absolutely."
Briles was booked on suspicion of child endangerment and resisting arrest. He was released Monday morning after posting bail.