I tried an experiment in the deepest part of a pool, when I was about 12 I think, I swam to the bottom and exhaled all the air from my lungs, slowly, to see how long I could remain on the bottom.
With no air,strength failing, and really wanting to inhale, I just made it to the surface, gasped a bit of air, and loads of water, and nearly sank again, I just summoned the strength to swim to the edge and grab a hand rail and pull myself up to splutter and spew water out and gasp in some air.
Ever since I have been careful, that really frightened me, silly boy that I was.
Good advice above from Glander re cold rivers, we recently had a case in the U.K where a guy dived in to a river to try to save a boy in trouble, and drowned himself, the boy was pulled out, his would be rescuer paid the ultimate price for his altruism, so sad.
Cold water anywhere can be a terrible danger, and currents, a moments thought, maybe grabbing a lifebelt or rope may ensure the rescue is a success.