The MH-65C helicopter crashed Tuesday evening near Point Clear, Ala. One crewmember was found unresponsive and later declared dead, the Coast Guard said.
The man who died was a rescue swimmer, said Capt. Don Rose, commanding officer of Coast Guard Sector Mobile. The three missing crewmembers were the pilot, the co-pilot and the flight mechanic.
Rose said rescuers tried to revive the rescue swimmer when they found him, but were unable to.
Names of the four crewmembers have not been released.
The crewmembers were outfitted with survival gear for the water, which was just over 60 degrees Fahrenheit overnight.
"These guys are wearing survival equipment, they're wearing what we call dry suits to protect them from the cold water," Rose said.
The accident comes less than three years after an HH60 Jayhawk helicopter crashed off James Island in Washington state in July 2010, killing three Coast Guard crew members.
Tuesday's wreck also brought back memories in south Alabama of a 1981 crash of a Coast Guard helicopter near an airport in Mobile that killed all four people aboard.
Chief Petty Officer John Edwards said divers overnight had gone to the site of the helicopter, in about 13 feet of water, but were unable to gain access to its fuselage. He said they planned to try again Wednesday with hopes of confirming whether the crewmembers were inside.
"The sun is up, which improves things greatly," Edwards said.
The aircraft had departed the Aviation Training Center in Mobile on a training mission, Bordelon said. Coast Guard officials say it went down about two to three miles southwest of Point Clear. The three missing are all Coast Guard members.
"Our thoughts and prayers are with the families of the crewmembers during this difficult time," Capt. Don Rose, commanding officer of Coast Guard Sector Mobile, said in a statement.
The Coast Guard said two helicopters, an airplane and several vessels were involved in the search.
National Weather Service officials said winds in the area Tuesday night were southeast at about 15 mph.