The dead crew member was identified as Executive Petty Officer Terrell Horne, age unavailable, the Los Angeles Times reported.
The deadly incident occurred near Santa Cruz Island while Horne and the other Guard member were investigating an open-hulled boat operating suspiciously without lights, the newspaper said.
The suspect boat rammed the Coast Guard boat and knocked the two occupants into the water. Horne suffered a deadly head injury, while the other Guard member had minor injuries.
"We are deeply saddened by the loss of our shipmate," said Adm. Robert J. Papp, the Coast Guard commandant.