Rapids.
I was a kid doing a week-long kayaking summer camp. Being that the students were young and kids, we weren't even taught to right ourselves, just how to "escape" if we flipped (pull the skirt off and swim free). Through the week we progress from flat water, to little ripple-like rapids, up to little 10-foot waterfalls. Then on the last day we do the big one, the "Devil's Triangle", where three currents meet at the same place and form strange spikey waves, leading into some massive churning rapids.
The night before we go it rains. A lot. The river - the whole thing, not just the hardest part that we're planning on doing - is closed to the public if it rises by 6 feet or more. It rises by 5 feet 10 inches. We go.
Like everybody else I go about 20 feet before I flip. It's OK, I know how to escape, I'm out in a second with my head above water. My paddle is gone. I cling onto my kayak for a bit, but it's being whipped around and in danger of clobbering me, so I let it drift away. I'm going down the rapids with nothing but my life preserver.
I swim for shore as hard as I can but I'm not even close to strong enough to make it out before being swept down-stream. I'm a fairly strong swimmer, but you'd need to be Michael Phelps to swim faster than this current. My feet point straight down as I'm tossed around - I was told later that that's a very dangerous posture, but the water was so deep it didn't matter.
After a minute of this I've made it to the shore (far downstream) and try to grab on, but my hands keep slipping. Finally a get a great grip on this large boulder, but I can't pull myself out. There is probably a foot of completely solid, not foamy, water just shooting around the rock, pushing me away from it with an incredible amount of force. As I sputter to keep my head out of the stream and keep breathing, I feel my grip slipping but I refuse to give in, I curl my fingers and pull harder and harder, my mind completely made up that I will stop not when my body tells me to, but when I'm out. But second by second I'm pushed back a little more, and after about 30 seconds my muscles give out and I'm swept back towards the center.
And then the part where I feared for my life. It looked like I was just going over a bulge of a few feet and into a bit of foam. It wasn't a bit, it was at least 10 feet deep and wider than I could reach of just foam. Foam kills you. It's light enough that it's impossible to swim in it, and dense enough that you can't breathe it. You can't move. I'm swept into a still spot in the middle of it, holding my breath, thrashing, unable to move an inch. The feeling that you can't affect whether you live or die is the most intense fear I've ever felt. I'm acutely aware of the steadily growing need to breath. I can't do a thing.
Before I run out of air, whatever slight current the foam has carries me out of it and I'm in water again, and rise to the surface.
The rapids die down after that. The water is still moving pretty fast, so I go down the river a ways before I make it to shore again, but I'm never completely tossed around like I was before. This shore is dirt, not rock, so I can swim into shallowed and less powerful water than I can get out of.