Marine on St. Croix or Marine on Saint Croix ( mÉ™-REEN on SAYNT KROY) is a city in Washington County, Minnesota, United States. The population was 664 at the 2020 census. It was founded in 1839 as Marine Mills. The city was the site of the first commercial sawmill on the St. Croix River. A substantial portion of the city is listed as a...