As rough-water swimmer Quinn Riordan trained in the wind-riffled, clear blue ocean off Kaimana Beach, he sliced through the water with barely a splash. Swift and efficient, leaving a clean ripple in his wake, he could have been mistaken for a blue-capped Hawaiian monk seal gliding across the channel.

After swimming recreationally much of his life, mostly in pools, Riordan developed a smooth, efficient style. But when he tried it in long ocean swims, he discovered he needed to adjust.

“Ocean racing requires a different form than pool racing,” Riordan said as he stood on the beach after his swim. “Swimming long distances in currents and waves, you don’t want to waste energy over-kicking. You want to pull with your arms and save your kick to hopefully accelerate at the end.”

Riordan, a 50-year-old retired Tokyo hedge fund president who grew up in Kansas City, Kan., and swam competitively in high school, splits his time between Hawaii and Japan. For the last 20 years, he and his family have regularly vacationed in the islands.

His swimming got a dramatic boost in September 2010 when he entered the Waikiki Roughwater Swim, a 2.38-mile race. He placed 199th overall and 19th in his age group, then 45-49.

Back home in Japan, he resolved to swim faster and started training for Oahu’s North Shore Swim Series the following summer. He even bought a house that Christmas on the slopes of Diamond Head, an easy walk to Kaimana Beach, where the rough-water swim starts.

The family returned to Hawaii earlier than planned, though, fleeing Japan after the March 2011 Fukushima earthquake and tsunami. Riordan, wife Rika, their children Takuma and Tanya, and his sister-in-law Yuki Tanaka and her daughter all headed for the islands.

Riordan faced new challenges as he adapted to ocean conditions, from high surf at Waikiki to a 10-foot tiger shark that swam up behind him during a swim across the Auau Channel between Maui and Lanai. Even navigating as he swims was new.

“It’s impossible to swim straight with waves and currents, and without a line painted on the bottom,” Riordan said, adding that sometimes the wind chop “feels like you’re being slapped in the face.”

The long distances involved in training can lead to injury, too. In order to avoid “swimmer’s shoulder,” a form of tendinitis, Riordan stretches before swimming and works out with weights in a program he devised with an online trainer.

Riordan believes that he’s avoided serious injuries because he spends a lot of time stretching and strengthening his shoulders and follows the correct swimming form — saving his shoulders by pulling with the lateral muscles in his back, keeping his chest down and butt and legs up.

“But almost every swimmer I know has had shoulder problems,” he said.

His dedication to ocean swimming has yielded gradual progress.

In 2011, the first summer Riordan entered Oahu’s North Shore Swim Series — three to five races off Sunset Beach, Waimea Bay and Haleiwa — he placed fourth overall. In 2012, the year he closed his hedge fund and retired, he advanced to third.

“Everybody improves as they spend more time training,” he said.

After placing second in 2013 and 2014, Riordan finally took first place this year in the 50-to-54-year-old group. But his times were faster than those of the winner in the 45-to-49-year-old group. Smart training has made him faster with age, Riordan said, noting he’s not alone.

“Looking at the 55-59-year-olds’ times, I would have gotten third in that division,” he said.

He has also steadily improved his times in the Waikiki Roughwater Swim, placing 142nd overall and sixth in his age group in 2014. (The event was canceled this year due to high surf.)

Swimming doesn’t seem to have an age limit.

“One of the most inspiring things (about rough-water swimming) is you see 80- year-olds still doing it,” he said.

Another nice thing: Although Riordan hasn’t lost weight, “a lot of this is coming up here,” he said, making a lifting motion from his belly to his chest. “I like to think so, anyway.”

His goal for next swim season is to defend his title in the North Shore series, which is far from guaranteed because some of his former age-group competitors are turning 50.

But there’s a special race in February. Quinn and Rika Riordan plan to enter the Honolulu Valentine’s Day Biathlon, a couple’s race: He will do the 1-kilometer ocean swim and she will do the 5K run. The baton pass is a kiss.

“Good Fit” spotlights inspiring fitness stories of change, self-discovery and challenge, and other fitnessrelated topics. Tell us what motivates you and how you stay fit and healthy. Email features@staradvertiser.com.