The cause of the crash, which happened about 100 miles southwest of New Orleans, isn't clear. Rovinelli did not know the type of helicopter or its exact destination.
He said the helicopter was carrying two pilots and seven passengers. Identities of the victims have not yet been released.
The National Transportation Safety Board has been advised of the crash and investigators were expected to arrive Monday, Rovinelli said.
Petty Officer Jaclyn Young said the Coast Guard assisted Terrebonne Parish sheriff's deputies in recovering the dead and rescuing the unidentified survivor from the marshy Bayou Penchant area. A sheriff's office spokesman did not return a phone call seeking comment early Monday morning.
Young said the survivor was being transported to an Ochsner Medical facility in suburban New Orleans. Young said the person was in critical condition.
Lafayette-based PHI is a primary provider of helicopter services to oil and gas platforms that dot the coast of Louisiana. It also flies medical helicopters.
Workers typically are flown to and from their worksites from coastal flight bases.
In June, a PHI Air Medical helicopter crashed in Texas, killing four. The accident in the Sam Houston National Forest killed the pilot, paramedic, nurse and a patient who was being transported from Huntsville to Houston.
That crew agreed to transport the patient after another helicopter company abandoned the mission saying that cloud cover was too low, making visibility poor in the early morning darkness.