Man, I've definitely been in some close calls in my time, but nothing tops my body's reaction to the damn great white that got way too close when I was out surfing.
I won't recount the whole tale, but suffice to say, I got a really good look at him, then saw him make a sharp turn and go under me. I was way out in the water, at the last break in a huge under-water drop-off at a deserted beach down a steep bluff. The sheer size of him was abstractly scary, and I could very clearly see his mouth hanging just a bit open, his white belly and dark top, the tail fin that was most definitely oriented the wrong way to be some freakish huge dolphin. He moved with such ease, and it was clear that I was helpless.
I always thought the term "time slowed down" was kind of a figure of speech. Nope. The very moment he went under me, everything was in terrifying super-slow motion and I could feel every beat of my heart against my chest as an individual event; I could tell when it was pumping, and filling. All the sounds of the ocean drowned down to almost nothing as if there were cotton in my ears but I could still make out the sounds around me. It probably took about 3-5 minutes to make it back to shore, but it literally felt like at least 15... 15 minutes of terrifying calm paddling the whole time knowing that if that shark wanted me, he could eat me right then and there, and there was nothing in my power I could do about it.
Yea, I've had some nightmares. No, I didn't stop surfing, although I haven't looked for excuses to go back to that same place, and haven't surfed there since.