The long, deep, cold lake that runs from Newport, Vermont across the Quebec line has carried reports of something large in it since the settlers arrived, and Abenaki tradition warned against swimming in it long before that. Sightings were being written down by the 1800s, and both Vermont and Quebec have passed resolutions protecting the creature, whatever it is, which may be the only lake monster in North America with legal protection on two sides of an international border. The lake itself is superb, over 300 feet deep in places.
Lake Memphremagog, Newport (Orleans County, Vermont)
See it on the map