At 3,300 feet (nearly 3/4 mile) long and 12 ft wide, the Coeur d'Alene Resort Floating Boardwalk is the longest floating boardwalk in the world and encircles the marina on Lake Coeur d'Alene. A 60 ft-long arched bridge provides access to the marina - and great views for pedestrians too. There's a bar at the west entrance, picnic and seating areas along the way, and the boardwalk is illuminated in the evening.
If you go:
The boardwalk is part of the Coeur d'Alene Resort, a luxury hotel, golf and spa complex, but the boardwalk is free and open to the public.
Links:

115 S. 2nd Street
Coeur d'Alene, Idaho 83814 USA
Phone Numbers:
The Coeur d'Alene Resort
+1 (855) 999-7998
+1 (855) 999-7998