The winter swimming season is already well under way in Helsinki. My Helsinki has put together a list of winter swinning spots that stay open throughout the winter and cut a hole into the ice when the water freezes over.
More info: https://www.myhelsinki.fi/fi/n%C3%A4e-ja-koe/aktiviteetit/7-x-talviuintipaikka-helsingiss%C3%A4