Swimmers have to monitor themselves vigilantly, too: their breathing, physical and mental states, and skin tone. “Hot pink is good,” said Jaimie Monahan, a top marathon swimmer from New York, who has taken to ice water swimming in the past few years. “Light blue, pale, that’s not good.”

Monahan, 37, completed an Ice Mile this past May in Reykjavík, Iceland, in 35 minutes in 37-degree water, entering a record book full of eye-popping figures. Last February, a man and woman in Ireland, swimming in 40 degrees, lasted 2.05 miles in 57:45 and 2.06 miles in 54:49. In December 2012, a pair of men in Russia each spent over an hour in 32.5-degree water swimming more than 1.3 miles.

The first question ice swimmers receive, then, is why.

As in many extreme sports, participants embrace the opportunity to learn about themselves. You may catch a surprising glimpse of some unfamiliar corner of your psyche. For a fleeting moment, you may experience the intoxicating feeling of defying your fundamental instincts. The pleasure of doing it seems to pale in comparison to the feeling of having done it.

“I’m being honest, when it’s over, the euphoria is incredible,” said Rena Demeo, who finished an Ice Mile two years ago in Boston.

For others, it has everything to do with the joy of competition.

On Saturday at the Wöhrsee, in the shadow of Burghausen Castle (of some renown as one of the world’s longest castles), a D.J. spun tunes — “Ice Ice Baby,” of course — as the events unfurled. Students selling baked goods behind a foldout table paused to swim short relays. The sauna at times seemed as packed as a rush-hour train car.

Ten minutes after winning her first race, Wittig dunked herself in the wood-fire hot tub behind the pier. Her shivering was so pronounced that she struggled to raise a cup of tea steadily to her lips. But by then she was smiling and laughing. After racing in her youth, she had spent 15 years out of competition. Now here she was, thriving in a different, quirkier sport. She talked about breaking the 13-minute barrier in the 1,000-meter race next month.

Wittig did four more races, four more plunges into the ice water, before the afternoon was done.