A simple utterance precedes a call to Yohei Yamada.

“Oh” — followed by one of two preferred English language expletives.

The utterer then typically watches the free-fall of a valuable object — sunglasses, a cell phones, car keys — until PLUNK!

They disappear into Lake Michigan.

Yamada, 42, is the guy who will dive in and return the object to daylight.

“I’ve got a really good success rate, so now everybody calls me,” says Yamada, who figures he gets about a dozen calls a week in summer’s peak.

Most of his work is confined to Chicago’s harbors and beaches, which he cruises in a 19-foot Boston Whaler. His dog, a rescue mutt named Emma, often accompanies him.

“I’ve seen it all. People drop a lot of things,” he said before ticking off a list.

“Divvy bikes, a Segway, grill lids, watches — one lady wanted me to find her sandal, it was Italian,” Yamada says.

Between his main gig of cleaning the underbellies of yachts and his side job as lost-and-found entrepreneur, he says he works 16-hour days seven days a week during the nonwinter months. He also captains private yachts in the evening.

He recently made the trip to Wilmette to hunt for a woman’s $13,000 gold bracelet that fell off while she was swimming. It took two days, but he found it.

Yamada texted her a picture of the bracelet in his palm.

“She was ecstatic,” says Yamada, who lives in Brighton Park. “That’s my favorite part — handing stuff back to the people.”

He charges from $100 to $200 an hour, depending on the degree of difficulty. Looking for objects in open water, for instance, is tougher.

He uses scuba gear — he says he also can hold his breath for four minutes — and works by hand in zero visibility, kicking up clouds of silt as he drags a mesh bag across the lake floor.

He finds metal detectors useless because Chicago-area harbors are littered with objects like bottle caps and screws, which result in false readings.

Yamada, who stays in shape by playing underwater hockey (Google it), was born in Tokyo but bounced around the world following his dad, who worked for Peoria-based Caterpillar. Stints in Illinois punctuated a childhood otherwise spent in Switzerland, Japan and Singapore, where he learned to dive.

In 1997, he moved to Chicago for college. But he also became a Chicago Park District lifeguard. And though he earned degrees allowing him to pursue a career as a physician assistant, he decided he preferred having Lake Michigan as his office.

Diving pays better, too, he says. And four months off from work in the winter allows Yamada to retreat to Green Turtle Key in the Bahamas with his fiancée.

But the trip is on hold this year. They’re expecting a baby girl.