8. The Sea Bed
If it's a sandy bottom it will feel like concrete if a wave pounds you down upon it. If it is a rock reef it might as well be concrete, although it will be much more jagged and likely to cause more injury that regular smooth concrete. If it is coral, you are in real trouble. First, it will cut you like a razor blade. Then it will leave tiny pieces behind to infect you for months after. Here's a tip; never, ever, fall off your board.