Nothing is impossible and it has also been said that Lake Champlain was once a saltwater sea (much larger) it is possible to have an evolutionary possibility of some strange type of prehistoric left over living creature. But, not likely! Another possibility is that Champlain is a very deep lake in areas and in those deep areas live very large sturgeons that occasionally clear the service of the lake searching for food. That scenario has been heard often.
It appears that the description of Champ is that of a dragon like creature that comes to the surface to breath. It seems logical if that is the case it would have a much higher witness rate than what has been reported. The deepest part of the lake is near the Burlington, Vermont area which is also the highest population area, so it would be likely that many views would have been seen if that was the case.