True eels do not grow anywhere near this size and the largest likely to be found in Lake Champlain would be around 6 ft in length.

People who have seen “sea serpents” in one place and then see something vaguely similar in another place are quite likely to guess it was the same thing each time. The largest serpentine object likely to be seen moving at speed on Lake Champlain, Loch Ness, Okanagan Lake or San Francisco Bay is going to be a flock of birds like geese or pelicans. I found this video of a “sea serpent” at :
http://www.lochnessinvestigation.com/oddities.htm

Poor viewing conditions could easily turn that into a 50 ft serpent swimming at 20 mph.