WAREHAM — The voice crackling over the radio was tense and urgent.

“Mayday, Mayday, Mayday. This is sailing ship Disco Volante at the entrance of the Cape Cod Canal, over.’’

The calm voice of a Coast Guard operator replied. “What is the nature of your distress?’’

The crew on the Disco Volante had spotted an overturned motorboat.

“There appear to be 11 people in the water. One child is missing, submerged under the boat.’’

It was 4:40 p.m. on Wednesday. The ocean was calm elsewhere, but in the notoriously dangerous mouth of the Cape Cod Canal, where the troubled boat had overturned, a stiff wind was blowing against the tide, kicking up a ragged four-foot chop.

The mayday call, sent over marine channel 16, the international distress frequency, crackled over ship radios all over the area. Responders dropped what they were doing. An MH-60 Jayhawk helicopter launched from a Coast Guard Air Station. The crew of a 45-foot boat from Station Cape Cod Canal gunned their boat’s engine. So did harbormasters from Wareham, Marion, and Bourne. In Wareham, a commercial diver known as “Diver Mike,’’ Michael Margulis, was wearing his wetsuit and inspecting a mooring in Sippican Harbor when he heard the call. As he raced his 21-foot motorboat, Island Time, toward the canal, a harbormaster’s boat blasted by him at full speed, lights flashing and sirens wailing. It was bad, he thought. And if a child was indeed trapped under the water, they had to move fast.

* * *

Aboard the Disco Volante, a 40-foot sailboat out on a training exercise, Martinus van Breems, the captain, had seen the boat struggle in the waves, take on water, and flip.

Now, nine people wearing life jackets — eight children and one adult — were about 25 feet from the Disco Volante, drifting away, pulled by the strong current. Perched on the black upturned hull of the 24-foot Four Winns Horizon, a man and woman were screaming: Their 8-year-old son was trapped underneath.

The Disco Volante maneuvered, and the crew started hauling people from the water.

* * *

By the time Lieutenant Matthew Bass, in his Massachusetts Environmental Police boat, and Adam Murphy, Marion’s deputy harbormaster arrived, the crew of the Disco Volante had pulled nine of the 12 boaters out of the whipping waves.

But the mother and father were still clinging to the top of the capsized boat, screaming that their child was underneath.

Bass, who was wearing his olive green uniform, dropped his heavy black duty belt, put on swim goggles, and jumped into the churning waters.

“It was terrifying, I think, for everybody,’’ he said.

Bass tried to swim under the upturned hull, but it was sunken several feet into the water and the waves were too rough for him to get underneath. Murphy was in the water now, too, and tried to hook a rope from the capsized boat onto his boat, to see if he could flip it right-side up. But it wouldn’t budge.

Marion’s harbormaster, Isaac Perry, had seen Margulis arrive. His boat was idling nearby and was already being swamped by the waves. Perry signalled him to move in closer.

“Do you have your dive gear?’’ Perry shouted.

Margulis shouted yes.

“Then get ready and look for a missing person in the hull,’’ Perry said.

Margulis, who is 56 and has been a certified diver since he was 17, pulled on a full tank of oxygen, adjusted his respirator, and plunged into the rocking waves.

He used his hands to feel his way through the darkness. He pushed away a pool noodle, a shoe, an empty life jacket. He was underwater about four minutes, methodically but desperately hoping to grasp a live person. Then he saw a flash of orange near his face. He grabbed it. Sure enough, the boy was wrapped inside. The life jacket had apparently pinned him to the floor of the boat.

Margulis cradled the boy under one arm and protected his head with the other, as he swam out from under the boat. He handed the child to Murphy and Bass who were clinging to the top of the hull. The boy was unconscious. Murphy and Bass began pumping the boy’s chest, as the mother and father watched from Bass’s boat, uttering words of hope.

Bass and Murphy quickly handed the child to rescuers aboard the Wareham harbormaster’s boat, which raced several miles to Tempest Knob, a pier in Wareham. There, the boy was loaded onto a stretcher and into an ambulance, which took him to Tobey Hospital in Wareham. From there, he was flown to a Boston hospital.

* * *

On Thursday, as the boy’s fate captured the area’s attention, boaters and workers on the docks in Wareham hoped against hope that rescuers had been able to bring him from the water in time.

Authorities said their best guess was that the boy was underwater for about 24 minutes, a length of time that stretches ordinary boundaries for survival.

Katie Gronendyke, spokeswoman for the Environmental Police, which is investigating the incident, said Thursday the boy remained in the hospital, but she had no word on his condition.

Authorities did not release the identities of the boy or the other boaters Thursday.

Margulis said he was impressed by the rescue effort, the speed of the response, and the determination of those who tried to save the boy.

“I just hope he’s strong enough to survive that ordeal,’’ he said. “He’s in God’s hands now. You can’t look beyond that.’’

Michael Levenson can be reached at mlevenson@globe.com. Follow him on Twitter @mlevenson. John R. Ellement can be reached at ellement@globe.com. Follow him on Twitter @JREbosglobe. Martin Finucane can be reached at martin.finucane@globe.com.