A red crescent moon hung over the horizon as I pulled up near Rio Vista Park, in San Marcos, just before six on a February morning. My car’s thermometer read 45 degrees. The park includes a short stretch of the San Marcos River, which is spring fed and thus stays at a constant 72 degrees. Though that would be pleasant as an air temperature, it means snorkeling in a wet suit with neoprene gloves, booties, and a hood. After I suited up, only my face was exposed. A hulking SUV pulled into the spot next to me, and bounding out of it, already zipped into his wet suit, came Nick Menchaca, the 35-year-old founder and owner of Atlas Environmental, a San Marcos–based business that works with…