Northern Wisconsin. Stopped for a picnic on a small island while canoeing down the Pelican River. The downstream point of the island, where the two branches of the river rejoined, was deep with a powerful current, and was always good swimming.
My brothers and I were diving off from the bank, seeing how far the current would carry us underwater before we had to take a breath. On my second dive, I dove a little too deep, and was pushed head-first into a rock. I was stunned a little, but otherwise unhurt.
No permanent damage.