CLEARWATER, Florida (CNN)  -- Two NFL players are among four boaters missing since Saturday night off Florida's Gulf Coast, officials told CNN.

Corey Smith, left, and Marquis Cooper are missing after a boating trip off the Florida coast, officials said.

Oakland Raiders linebacker Marquis Cooper and NFL free agent Corey Smith, who played for the Detroit Lions for the past three seasons, have not returned from a Saturday fishing trip in water off Clearwater, Florida, family members and colleagues said Sunday.

"It feels like my greatest fear coming true -- it doesn't feel real," Cooper's wife, Rebekah, told CNN affiliate WTSP in Tampa on Sunday afternoon. "I'm just waiting for a phone call."

The Coast Guard began searching around 2 a.m. Sunday after it learned that four men -- Cooper, Smith, and former University of South Florida football players William Bleakley and Nick Schuyler -- had not returned from their fishing trip Saturday evening as expected.

Rough weather has hampered the effort, but "it's still a very active search," Coast Guard Capt. Tim Close told reporters Sunday afternoon.

Poor visibility from heavy waves forced the search-and-rescue effort to be conducted primarily by air, though authorities deployed patrol boats as well, Close said.  Watch relatives, friends express concern for missing boaters Â»

Searchers have focused on 750 square miles of open water in the Gulf of Mexico, about 50 miles west of Clearwater Pass. From there, the men had left the Seminole Boat Ramp in a 21-foot single-engine boat about 6:30 a.m. Saturday, the Coast Guard said.

Rebekah Cooper said she became worried Saturday night when she didn't hear from her husband. She called her one of her husband's fishing buddies, Brian Miller, who contacted the Coast Guard with the coordinates of where the men planned to fish.

"Usually I'm on the boat. It's a little difficult wondering if something would have been different if I had been there," Miller said. "Or who knows? They may be just sitting out there with a broken motor -- and that's what we're hoping for."He said it was clear something was wrong when Cooper didn't call Saturday night.

"He should've been within range to use his cell phone, and he knows enough to shut it off when he goes out so the batteries are still there," he said.

Close said weather conditions were relatively good Saturday, "but the weather picked up overnight."

"It's a small vessel for the conditions that are out there right now," Close said Sunday afternoon.

Rebekah Cooper said her husband was aware of Sunday's weather forecast and for that reason picked Saturday for the trip.

"Fishing is his first love, it always has been," she said, adding, "I have a lot of faith in him out there."

Cooper's father said he learned of the situation Sunday morning from his daughter-in-law. His son "routinely stays out on the water 12-14 hours," Bruce Cooper, a sports anchor for CNN affiliate KPNX in Phoenix, Arizona, said in a statement.

Close said authorities had not received a distress signal from the boaters. Close said Cooper owns the boat.

Bruce Cooper called his son an "avid fisherman."

"He goes deep sea fishing any opportunity he gets," Cooper said in the statement. "Two years ago I went deep sea fishing with him. I swore I would never do so again; I didn't like the fact that I couldn't see land. Needless to say I am very concerned. I am praying and hoping for the best."

The boat, a center-console vessel manufactured by Everglades Boats, is billed as "unsinkable," Close said.

The Detroit Lions released a statement acknowledging that Smith was among the missing men, adding, "Our thoughts and prayers are with all the passengers, their families and all those involved in the search efforts."

Smith and Cooper were
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question: What NFL players are missing?, answer: Marquis Cooper | question: Where did four people go missing?, answer: off | question: What is theorized about their disappearance?, answer: greatest fear coming true | question: What feels like the greatest fear coming true?, answer: Oakland Raiders linebacker Marquis Cooper and NFL free agent Corey Smith, who played for the Detroit Lions for the past three seasons, have not returned from a Saturday fishing trip | question: When did the boat set out?, answer: 6:30 a.m. Saturday, | question: Off of what state's coast did these people go missing?, answer: Florida | question: What football players are missing?, answer: Marquis Cooper | question: Who are the among the missing?, answer: Two NFL players | question: What sports league were Corey Smith and Marquis Cooper associated with?, answer: NFL | question: How many boaters are missing>, answer: four | question: What agency was helping to find these people?, answer: Coast Guard