Zhuang Ni, a Chinese instructor, teaches an introductory course of freediving in Ying Tung Natatorium in Beijing. [Photo by Xing Yi/China Daily]

Many freedivers attracted to the lifestyle have turned it into a career. Apart from those who become instructors, Li Da has become what is known as a mermaid. This refers to underwater models in specially designed mermaid fins, and Li is one of those in China most well known for that role. She founded Dada Li Freediving and Mermaid Academy in Shenzhen, Guangdong province, in April.

"Depth is not what I dive for," Li says. "I prefer to enjoy the underwater moments that leave me with beautiful images."

Li, who used to work for a Norwegian shipping company in China, first learned freediving in 2012, and in subsequent years has used all her holidays to travel to islands to do freediving.

Li once spotted photos of mermaids online and decided to make her own mermaid fins. Last year, photos online of her swimming as a mermaid in the western Pacific country of Palau were widely circulated among diving aficionados.

"The photos amazed many, and I got people wanting me to teach them to be a mermaid," says Li, who became a certified instructor last year.

"Many people, especially women, learn freediving so they can take beautiful pictures."

Her company offers courses priced from 1,500 yuan.

"I throw in a few things from dancing into freediving and teach students techniques of underwater posing."

This year Li has recruited and trained a team of 10 mermaids, creating a 10-minute underwater play about mermaids and princes for a newly opened aquarium in Guangzhou. The show, performed daily, has become highly popular among visitors to the aquarium.

Last month Li created a new play in which the actors are dressed as super heroes such as Superman, Batman and Catwoman.

"At first freediving was just a bit of fun for me," Li says. "Now I teach others how to get fun out of freediving... The joy on the face of my students, just as I first experienced it, is what keeps me teaching it."