Lake St. Croix Beach or City of Lake Saint Croix Beach is a city in Washington County, Minnesota, United States. The population was 1…Lake St. Croix Beach or City of Lake Saint Croix Beach is a city in Washington County, Minnesota, United States. The population was 1,043 at the 2010 census. According to the United States Census Bureau, the city has a total area of 0.99 square miles ; 0.55 square miles is land and 0.44 square miles is water. County 18 serves as a main route.