Gordon found this cool article about a giant northern water snake, which, apparently is large enough to eat big catfish. This is my favorite part:
Northern water snakes are distinguishable from their venomous look-alikes by their pupils: cottonmouths have a vertical slit pupil while Northern water snakes have a round pupil, Brown said.
If I see a snake swimming toward me, I am going to swim away as fast as I can. Who is going to look at its pupils? Here, let me lean in real close to see if its pupils are slit and it might be a cottonmouth, whose venom is more toxic than that of a copperhead. This way my nose will provide a nice convenient target.
This reminds me of another useless advice I once read. If you see a canine in a forest and you are not sure if it’s a wolf or a dog, observe how it drinks. If it sucks the water in, it’s a wolf, if it laps it, it’s a dog.