Just got off a dive boat in SoCal. On the first night dive, a buddy and I were hanging on the anchor line waiting for a third diver (for about 10 minutes, it turns out.) My buddy got whacked in the mouth (cut upper and lower lips, decent bruising, hurt ego from everyone else laughing...) by a flying fish who was trying to escape a sea lion. Note: if fleeing from a predator, knocking yourself out on a nearby object is generally not going to help.
The next night, I scared up a lobster which fled my vicinity, and headed for my buddy, 30 feet or so away. (I followed the lobster as it went over a little sand dune about 25 feet away, then all of a sudden, Steve's light waved wildly...) Nailed him in the chest, he said. I almost drowned, I was laughing so hard.