In my 20's, when we were invincible, my friends and I would water ski, wakeboard, and kneeboard. Unfortunately, we also would do so without a spotter, because we didn't always have enough people. We developed a game where we'd put out 2 tow lines of equal length, and 2 of us would have "kneeboard wars" trying to knock the other down hand-to-hand combat style. We weren't stupid, we only did it on kneeboards, where we didn't have as far to fall as when we were on skis or wakeboards! Anyway, we were out with just the 3 of us, and 2 of us were warring, with just one in the boat at the helm. Let's call him #1, and it was his boat. He had just put a Doel Fin or equivalent on the outboard, and loved the fact that he could now turn sharply at higher speed without the prop ventilating and losing bite. But it also allowed the boat to exhibit a very bad tendency to "catch an edge," which I think might have been caused by a spray channel along the chine (late 60's round chine aluminum AMF runabout.) When this happened, it was usually a pretty aggressive "toss" of the boat, and instigated a tighter turn. So, me and #2 were laying across our kneeboards, waiting for #1 to bring the boat around between us, and we were facing in the direction we figured he'd pull us back up. Without looking backwards, I thought #1 was running at too high a speed, obviously making the turn quickly before slowing down. Well, out of the corner of my eye over my shoulder, I saw the cream color of the bottom of the boat. I was like "WTF!!!!" and like kinda kicked against the boat. Then my back and heel of foot were numb, and the boat was stopped. #2 was frantically swimming to me, and #1 was looking over the side of the boat. They were yelling at me "Wiggle your feet!!!!" and I was yelling at them "AM I CUT, AM I CUT" frantically trying to look back and see if there was any red water. We calmed down, and I wiggled my damn feet. And #2 looked over my back and didn't see any cuts or blood. I said I climbed into the boat, but they said they bodily lifted me. We went immediately back to my Grandparent's place, and they physically pulled me out of my full length wetsuit (was October or something) and we packed into the truck for a trip to the hospital. Told my Grandparents I had twisted my back on the wakeboard, but Granddad was no dummy and knew we had done something stupid. ER x-rayed me, and gave me muscle relaxants to deal with the muscle spasms. Next morning they called me and told me to call my doctor. Turns out the radiologist saw hairline cracks in L3 and L4 transverse processes.
So, basically, what happened was #1 was turning at too high a speed, intending to go between us laying on our kneeboards. I was to the right, and the boat was turning to the right, and I expected it on my left side. The boat tripped a chine, and knocked him out of the driver seat, and tripped a chine again. He clawed to the throttle just in time, but now the boat was turning a tighter radius and going to my right side. The stern of the boat was sliding out, and this is what basically mashed me across my back. Good thing I was wearing a wetsuit and PFD, which protected my upper back. I never got close to the spinning prop, which, thank god, because my greatest fear as a child was getting cut up by a propeller. Well, and getting my feet bitten by crabs. So, I spent a week on muscle relaxants (basically sleeping) and then a month not lifting anything, and then I was good to go. Have had no problems since.
But, christ, feeling that boat hit me, and not knowing if I was cut open by a prop... I've had some other scary boating things, but this probably takes the cake.