The Fresno County Sheriff's office says the deadly accident happened shortly before 8 a.m. Friday.
Investigators say 35-year-old Matthias Aguilar of Merced was fishing in a kayak on the reservoir when he fell into the water.
He attempted to swim to shore before going underwater.
Friends took him back to the shore and performed CPR.
Aguilar died by the time deputies and a Skylife helicopter arrived on scene.