Joseph Martinez, 28, was the coach of the boy's and girls swim and water polo teams at Mission San Jose High School, said Kirsten Diktakis of the Fremont Unified School District.
Martinez died at an area hospital on Monday. An autopsy was performed Tuesday, but the cause of his death has yet to be determined, according to the Santa Clara County medical examiner's office.
Martinez had coached at Mission San Jose since 2003, and was also the coach of the Mission Valley Barracudas swim club and the water polo team at Ohlone College, Diktakis said.
The district has provided grief counseling services at Mission San Jose High School this week for students affected by his death, Diktakis said.
A Facebook page titled "Joseph Martinez was my friend/water polo/swim coach" popped up in the wake of his death, with more than 100 people posting messages of memories they had of Martinez and shock at his passing.
"Joe, the pool decks of the Fremont area will seem empty without you," said one of the messages.
Diktakis said the death occurred in the middle of the swim team's season but that the team's other coaches are continuing on, despite the loss of Martinez.
According to the Facebook page, Martinez's memorial service is scheduled for Saturday in San Jose.