Donning her custom-made tail, handcrafted from recycled wet suits and eco-friendly paints, Kari Roberts is accustomed to getting strange glances on the shoreline as she embarks on an ocean swim. Nowadays, she’s been spending some time in the pool, teaching the "Fin to Fitness" swim class at the JW Marriott Ihilani Resort & Spa in Ko Olina.

"I’ve always been a mermaid fanatic. I’ve swam with manta rays, sharks and dolphins (wearing her tail)," said Roberts.

At the age of 4, she dreamed of having a mermaid tail after watching the Disney classic, "The Little Mermaid," and dressing as a mermaid became her go-to Halloween costume over the years. But she never imagined she’d be able to parlay her mermaid obsession into a career.

Roberts — who uses the name Mermaid Kariel at work — now owns two tails, in pink and blue, each taking more than 300 hours to construct.

The "Fin to Fitness" class, which is open to the public, was a perfect fit for the Ihilani Spa’s "Healing by the Sea" theme, according to spa director Robin Desha. "The workout is great for both men and women, as it is designed to utilize the whole body with concentration on the core — abdominals, back, legs and butt — for a sleeker physique.

"I saw Kariel swimming and thought the dolphin swim would be a great water aerobics class. The dolphin swim is used to train for the Olympics, so I knew it was genuine."

The class, taught in the spa’s lap pool, works every muscle group, according to Roberts. Participants learn the dolphin kick, a competitive underwater swimming technique that emulates the way a dolphin cuts through the water with its flipper. Each session burns 300 to 700 calories per hour, depending on intensity, and is designed to improve balance, coordination and agility.

During the first half of the class swimmers warm up on the pool deck with stretches and receive instruction on proper swim technique. The second half of the class is spent in the pool, where participants wear a monofin and swim underwater with the help of Roberts and her assistant. Masks are available to wear.

"The program is gentle, yet effective. The water provides resistance, which tones the body. It’s good for everyone, including pregnant people, the elderly or people with fragile joints," she said. "People become more flexible and can gain a lot of strength. Flexibility is increased, especially in the torso, ankles and arms, because you are moving in a worm-like motion. It’s a cardio workout, not something that’s just fun and cute."

Roberts is certified in first aid and CPR and is also a PADI (Professional Association of Diving Instructors) advanced open-water diver and an aquatic fitness professional trainer.

Oregon visitor Liz Ketler, 54, said she decided to try something different during her stay and swim with a mermaid. "The difference with this class is that the exercise feels painless and is not hard on your body because you are weightless in the water. It is a great workout for the core muscles and lung power."

Lark Reece, 58, also from Oregon, purchased her own monofin to perfect her dolphin kick. "I belong to a club that has a wonderful indoor pool. I love the low-impact, high-burn effect of such a workout," she said. "Kariel has a grace and presence that is totally likable, so sweet. She’s the perfect mermaid."

In addition to the fitness benefits, Kariel said participants "get that inner-child experience where they get to swim with a mermaid and pretend to be one."