Tossed from his boat on Friday morning by an errant wave, Justin Underwood feared not for himself but for someone his unmanned and out-of-control powerboat might injure.

“My first worry was if it was going to hit someone else, someone on a dock, or a house near the water, or something like that,” he said.

“Kind of like the first minute, I was pretty angry at myself ... then I just focused.”

Underwood, 21, of Bayport, who had headed out in the rain and fog shortly after 7 a.m. to his summer job, had an abundance of experience to draw on.

Not only has he grown up around the water, he’s a surfer and a lifeguard, swimming instructor and assistant manager at the Patchogue Beach Club.

His usual 10- to 12-minute commute was interrupted when he briefly took his hand off the steering wheel, which was slippery in the rain, to consult the compass on his cellphone. The bay was choppy, and that is when the wave rocked the 2011 19-foot NauticStar, tumbling Underwood into the drink off Blue Point.

Underwood, who had not donned a life jacket, figured he was about 2 miles from land.

advertisement | advertise on newsday

“I couldn’t see the shore.”

But he had consulted the surf conditions before heading out, and knew the wind, mainly blowing north, was the direction to swim toward.

The rain and a few gulls served as telltales.

“There were a couple of sea gulls flying above me; they just hover, facing into the wind, and I knew it wasn’t supposed to shift at any point.”

About 10 minutes after his long swim began, his employer texted him to tell Underwood he could skip reporting to work because of inclement weather.

So his boss would not have been alarmed when he didn’t arrive on time. And his cellphone had stayed on the boat.

Fortunately, the water temperature was about 80 degrees; experts say that is around the ideal temperature for swimming laps.

“It’s interesting to actually practice what I preach,” said Underwood, who teaches children ages 4 to 12 to stay calm, and swim energy-conserving strokes, like the breast stroke and sidestroke, if a long swim in open water is all that will save them.

A 911 call reporting the unmanned boat was received at 8:06 a.m., police said.

Marine Bureau officer Sgt. John Vahey, aboard Marine Charlie, found the abandoned boat between Brown’s River and Homan’s Creek a short time later.

advertisement | advertise on newsday

The NauticStar was recovered about 10 minutes later after three other Marine Bureau officers, Raymond Cairo, Dale Kelly and Raymond Hawkins, jammed the propeller with a life ring and line, police said.

Once slowed, Kelly jumped aboard and brought the unmanned vessel under control.

Underwood estimated he’d been swimming about an hour and a half and was half a mile offshore when he spotted the marine police searching for him in the fog.

He used the dark blue shirt he was wearing to flag them down, but it was not bright enough.

“It wasn’t a great feeling when you see them doing their scanning patterns and stuff looking for you and not finding you.”

advertisement | advertise on newsday

So he removed the bright red bathing trunks lifeguards wear and used it as a flag, ending a tense 20-minute search.

After they spotted him, he put his bathing suit back on. Vahey pulled him out at 8:36 a.m.

The police rescued him near Buoy 35 in the North Channel, south of Blue Point and about 1 mile from where his boat was found.

After Friday’s morning adventure, Underwood said he ate a big breakfast and downloaded a U.S. Coast Guard safety app.

Underwood, who will be applying to medical schools when he returns as a senior to the University of Delaware this autumn, has demonstrated his talent for remaining calm during emergencies at least once before.

Last year, he and fellow lifeguard Nick Costantino were honored for saving a 5-year-old boy who had nearly drowned at the swim club. The two emergencies were not dissimilar.

“It’s kind of like the same thing, like that situation; it’s kind of like your adrenaline’s pumping.”