I was swimming in a Canadian glacier lake (they're really deep, icy cold, and surrounded by granite shores) with some other Luther-League kids. About 500 miles from home, three hours' boat ride to the nearest town, another couple of hours' drive to a hospital. The canoe tipped over as they so often seem to do with teenagers. It was near shore, but it was also over a serious drop-off to some 30 or more feet of depth. DARK water, because it's so deep. The girls squealed and swam to shore, while Jim (yes, that Jim) and I laughed at them and tread water.
Then something rose from the deep. There I was, treading water several body-lengths from the murky lake-floor, when suddenly my foot landed on what felt like a rock and I was able to stand on it and rest for a moment. Only a moment, because even as a teenager I was able to add 2+2. How could a rock exist at this place, where the lake-bottoms are scoured smooth by glaciers the size of mountains? And then the rock sunk beneath my weight, and something scaly and sharp rasped agaist my calf.
I think I ran across the surface of the water to the shoreline in less than a second. Jim climbed atop the overturned canoe. A kraken's head rose briefly from the waters, its ancient eyes surveying its surroundings, calculating whether we should be prey or not. Then it sunk back beneath the inky waters.
(Well, perhaps not a kraken, but have you ever seen one of those mammoth snapping turtles? Beaked head the size of a Rottweiler's, claws the size of a grizzly's, shell the size of a VW Beetle.)
I think it was about then that I stopped enjoying a swim in the deep quite as much. If you can't see beneath you, it's just not as fun. If you're wearing a swim-mask, you can look down and see your enemy. The only remaining question is, "Can I swim to safety before it attaches itself to my thigh?"