It's Michigan. It has the longest freshwater coastline in the US and the second longest coast line in the US next to Alaska.
There is a comparative table of state shorelines on the Michigan Dept. of Environmental Quality's site that gives the measurements for states touching the Great Lakes. See http://www.michigan.gov/deq/0,1607,7-135-3313_3677-15926--,00.html.
According to World Book Encyclopedia (v.13, p.500 of the 2000 edition), Michigan's shoreline, at 3,288 miles is "more than any other state except Alaska. This includes 1,056 miles (1,699 kilometers) of island shoreline." In v.1 (p.284 and 294) of the same edition it states that Alaska has 6,640 miles or 10,686 kilometers of coastline.
More Michigan facts and trivia? Get them on 50States.com.
Michigan is home to more than 100 public, some of the highest freshwater sand dunes in the world, stunning multi-colored sandstone cliffs, two National Lakeshores and the only national marine sanctuary in the Great Lakes--the Thunder Bay National Marine Sanctuary in Lake Huron.
The state also boasts more than 100 lighthouses, numerous maritime museums, ten shipwreck-diving preserves and historic military fortifications rim Michigan's Great Lakes shoreline.
Explore www.michigan.org to find out what pure feels like in the waters of Pure Michigan.