The accident happened near Orwood resort, which is north of Discovery Bay, during a state Bass fishing championship qualifier.
About 50 teams registered from across the state.
First responders say one boat crashed into another.
"We have a lot of 5 mph zones here on the Delta, so I don't know if they went to slow down for the idle zone and the other boat didn't, or if they turned in front of them, or what happened," Brentwood resident Jared Defremery said. "But that's what I said, you just got to be aware of what people around you might do because it's not just us out here driving alone."
Organizers say the crash happened during the initial run as the boats were heading to their first fishing spots.
The collision remains under investigation.