You really shouldn’t be able to swim here. Set on barren shores in the northern reaches of the North Atlantic, the surrounding waters are nothing short of frigid, normally forbidding people from even dipping a toe into the cold sea. But still, there’s a beach. And people paddling around in the water. Pulling up my swimming trunks and drawing a deep breath, I wade in, forcing myself deeper, step by step, until I’m completely submerged. While ocean temperatures off Reykjavik, even on a balmy June day, usually hover somewhere in the mid-40s Fahrenheit (dropping into the 30s during the winter), here at the Nautholsvik Geothermal Beach, you can take a dip all year round. With waters warmed by the clean runoff from a nearby natural energy plant, you can enjoy a fairly comfortable plunge in this man-made lagoon, with hot tubs and steam rooms awaiting you, once you’re done with …