I've heard, but can't verify, that snapping turtles are only aggressive on land and not in the water. Reason being that, unlike most turtles, they can't tuck completely into their shell for protection. So on land they're vulnerable so they have to be aggressive to stave off predators. In the water they can simply swim away.
I helped snapping turtles cross the road twice. The first time was a smallish one and I grabbed it on either side of the shell at the widest point figuring I was safe. About half way across the road it snapped its neck out quick as a rattler and just about got my finger. I dropped it and carried it by the tail the rest of the way.
The next time the turtle was bigger and I used my paddle to help it along and flip it up over the curb. It pissed something stinky all over the paddle which interested Sadie very much when I got back in the car. Whatever it was she must have given it a little lick. When she was done inspecting the paddle she jumped back into the front seat and I looked over to see her mouth covered with foam and drool running out all over the place. Apparently that's another one of their defense mechanisms.
Now when I see a snapper crossing the road I usually just wave and say, "good luck".
Alan