Mark O’Leary wasn’t sure if he was going to go into work Friday morning. But plenty of people are glad he did.

The 19-year-old has been a lifeguard in South Boston for the past four years and spends much of his time on duty planning his reaction to various scenarios a guard could face on Pleasure Bay.

But around 3 p.m., O’Leary faced a real-life emergency.

A man was face-down in waist-deep water, doing a “dead man’s float,” and lifting his head up occasionally, O’Leary said. He continued to watch the man, and a few children near the shoreline.

When the man didn’t lift his head after a few moments, O’Leary went down to the water with his CPR mask and rescue tube in tow.

“I kind of propped him up, and that’s when I realized he was unconscious,” he said.

O’Leary yelled for the other beachgoers to call 911 as he dragged the man out of the water.

“There was foam coming out of his nose, and I couldn’t feel a pulse,” he said.

On shore, O’Leary did one compression on the man’s chest and realized that he was trying to breathe on his own. O’Leary turned him on his side and the man coughed up water, he said.

He could feel a slight pulse and kept the man in the recovery position until an ambulance arrived, O’Leary said.

State Police responded to the scene to assist Boston EMS. The victim was a homeless man who is expected to recover after the near-drowning, State Police said in a prepared statement.

S.J. Port, spokeswoman for the state Department of Conservation and Recreation, said the man is in his 40s.

Port said that as the summer winds down, many of the DCR lifeguards are going back to college. She encouraged swimmers to find beaches that have guards and to always have a buddy.

She said beachgoers should refrain from using alcohol or drugs when at the beach and urged people to alert DCR staff or the police if they notice a dangerous situation.

“We want to make this one last summer weekend a safe one,” Port said.

The people with the man at the beach were grateful that O’Leary decided to put his red shirt on that morning.

“They just kept saying, ‘Thank you, thank you,’ ” he said.

Only four lifeguards were on duty at the bay, and the second guard assigned to Station 2 with O’Leary was on break.

That meant he had to face the situation alone. O’Leary has passed out countless bandages for minor cuts and picked children out of the water, but had never had to rescue an adult.

“I’m pretty proud of myself,” he said. “Every day you think of what you’re going to do if something goes wrong. I just kind of knew.”

O’Leary’s parents are pretty proud of their son, too. He said they were grabbing some food at Sullivan’s, Castle Island’s iconic burger joint, when they noticed an ambulance race by.

“My mom called me to find out what was going on,” he said. “She didn’t know it was me who made the rescue. She was pretty happy.”

O’Leary is a sophomore at Westfield State University and thought about skipping his lifeguard shift since he is moving into an off-campus house with his friends on Saturday.

He said, “There’s a lot I have to do,” and he planned to “pack and get ready to go” as soon as his shift ended at 5:30 p.m.