That's odd that they would be a significant sighting in Algonquin since they're common all around the park. In any event, I have fished and been an astute and frequent observer of nature for over 40 years, so I can confidently say what I didn't see, and the only two things I can't rule out are american eel and mudpuppy. It sounds to me like the latter is more probable, although the former is what they really looked like to me.