http://en.wikipedia.org/wiki/Mercury-Redstone_4#Hatch_blows_open

Hatch blows open

After logging the panel data, Grissom asked the helicopters to begin the approach for pickup. He removed the pin from the hatch-cover detonator and lay back in the couch. "I was lying there, minding my own business," he said afterward, "when I heard a dull thud." The hatch cover blew away, and salt water swished into the spacecraft as it bobbed in the ocean. The Liberty Bell 7 began taking on water and was sinking fast.

Grissom had difficulty recollecting his actions at this point, but he was certain that he had not touched the hatch-activation plunger. He removed his helmet, grasped the instrument panel with his right hand, and climbed out of the sloshing hatchway. Floating in the sea, he was thankful that he had unbuckled himself earlier from most of his harness, including the chest restraints, otherwise he might not have been able to exit.

Lieutenant John Reinhard, co-pilot of the nearest recovery helicopter, reported afterward that the helicopters were making their final approach for pick-up. He was preparing to cut the spacecraft's antenna whip (according to a new procedure) with a squib-actuated cutter at the end of a pole, when he saw the hatch cover fly off, strike the water at a distance of about 5 ft (1.5 m) from the hatch, and then go skipping over the waves. Next he saw Grissom's head appear, and the astronaut began climbing through the hatch. Once out, Grissom swam away.

Failed spacecraft recovery

Leaving aside the swimming astronaut, Lewis completed his approach to the sinking spacecraft, as both he and Reinhard were intent on spacecraft recovery. This action was a conditioned reflex based on past training experience. While training off the Virginia beaches the helicopter pilots had noted that the astronauts seemed at home in and to enjoy the water. So Reinhard quickly cut the high-frequency antenna as soon as the helicopter reached Liberty Bell 7. Throwing aside the antenna cutting device, Reinhard picked up the shepherd's hook recovery pole and carefully threaded the crook through the recovery loop on top of the spacecraft. By this time Lewis had lowered the helicopter to assist Reinhard in his task to a point that the chopper's three wheels were in the water. Liberty Bell 7 sank out of sight, but the pickup pole tangled as the attached cable went taut, indicating to the helicopter pilots that they had made their catch.
Reinhard immediately prepared to pass the floating astronaut the personnel hoist, but at that moment Lewis called a warning that a detector light had flashed on the instrument panel, indicating that metal chips were in the oil sump because of engine strain. Considering the implication of impending engine failure, Lewis told Reinhard to retract the personnel hoist while he called the second chopper to retrieve the pilot.

Meanwhile, Grissom, having made certain that he was not snared by any lines, noticed that the primary helicopter was having trouble raising the submerged spacecraft. He swam back to the spacecraft to see if he could assist, but found the cable properly attached. When he looked up for the personnel line, he saw the helicopter start to move away.

Suddenly, Grissom realized that he was not riding as high in the water as he had been. All the time he had been in the water he kept feeling air escape through the neck dam. The more air he lost, the less buoyancy he had. Moreover, he had forgotten to secure his suit inlet valve. Swimming was becoming difficult, and now with the second helicopter moving in he found the rotor wash between the two aircraft was making swimming more difficult. Bobbing under the waves, Grissom was scared, angry, and looking for a swimmer from one of the helicopters to help him tread water. Then he caught sight of a familiar face, that of George Cox, aboard the second helicopter. Cox was the copilot who had retrieved both the chimpanzee Ham and Shepard on the first Mercury flight. With his head barely above water, Grissom found the sight of Cox heartening.

Cox tossed the "horse-collar" lifeline straight to Grissom, who immediately wrapped himself into the sling backwards. Lack of orthodoxy mattered little to Grissom now, for he was on his way to the safety of the helicopter, even though swells dunked him twice more before he got aboard. His first thought was to get a life preserver on. Grissom had been either swimming or floating for a period of only four or five minutes, "although it seemed like an eternity to me," as he said afterward.

As the first helicopter moved away from Grissom, it struggled to raise the spacecraft high enough to drain the water from the impact bag. At one point the spacecraft was almost clear of the water, but like an anchor it prevented the helicopter from moving forward. The flooded Liberty Bell 7 weighed over 5,000 pounds (2,300 kg), 1,000 lb (450 kg) beyond the helicopter's lifting capacity. The pilot, watching his insistent red warning light, decided not to chance losing two craft in one day. He finally cast loose, allowing the spacecraft to sink swiftly. Martin Byrnes, aboard the carrier, suggested that a marker be placed at the point so that the spacecraft might be recovered later. Rear Admiral J. E. Clark advised Byrnes that in that area the depth was about 2,800 fathoms (5.1 km).