Funny you should ask. I saw one of these for the first time at a local lake recently and took a picture as well. For all intents and purposes, as far as I can tell it looks like an aquatic praying mantis (does that even exist?). Its got the same shape of head as the garden variety ones around here, and the forward swept arms have definitely got folded "pinchers" on them. Whatever they are, they were damn cool.
Yea this is a water scorpion. There are two water scorpions in North America, they both bite and both hurt alot, but this one is the nastiest of the two because it's poisonous just like Stephen Rice said. Hopefully you steered clear of this guy!
They belong to the family nepidae (I had to memorize all these names in entomology). They're one of my favorite aquatic insects, There chelicerae (I think that's what they're called) remind me of mantis shrimp, and like mantis shrimp, they are very willing to hurt you.