Want to swim like a fantasy sea creature? At 'mermaiding' classes in Finland, adult women and men swim wearing a garment that combines a monofin flipper with fabric from the waist down to create a distinctive mermaid tail.

"It's my childhood dream-come-true. When I was a kid, I always wanted to be a mermaid or a dolphin but I think it's easier to become a mermaid because it's half human," Finland's first professional mermaid instructor Maija Mottonen, 28, told Reuters on a recent visit to one of her classes at a pool in Espoo, on the outskirts of Helsinki.

Her ornamental pink mermaid tail and the pool's ocean-like blue colour make it hard to remember that outside it is a blistering cold winter day with the ground covered in snow.

Since August, Mottonen, a former kindergarten teacher, has taught more than 200 mermaids and mermen how to swim with a tail on, one of them being 29-year-old construction worker Markus Parviainen.

"It's kind of magical or something, he described the feeling of diving with the green tail that he had ordered online for some 170 euros ($195).

Parviainen, who is also into cheerleading, said his merman hobby had raised some eyebrows among his coworkers when he first started but has paid little attention to what others think.

"People think, or mostly men, are thinking that this is only for girls or women but I disagree, this is for everyone... as long as you love swimming," he said, adding it was an efficient way for keeping fit.

Mottonen teaches her pupils how to undulate their entire body from head to toe, "like a snake" as she described it, followed by a dolphin kick with the monofin flipper to move forward.

More advanced merpeople like Parviainen practice tricks such as flapping their tail on the surface while hanging with their head straight down towards the bottom.

"When I had tried this once, I was hooked. I feel real powerful in water," another pupil, Annika Ihatsu said, having dared a 40-minute drive on the icy roads from her hometown Hyvinkaa to attend the class.

While Finland may not be the first place on earth to cross people's minds for fairytail swimming, instructor Mottonen said in summertime she preferred the Finnish sweetwater lakes to practise over warmer beaches in other countries where salty seawater would make her tail float.

"We have a lot of lakes so it's easy in summer," she said, referring to the country's roughly 180,000 lakes.