A friend of mine went under water for half a block when the edge of a sand bar unexpectedly broke off under his weight. It threw him into current and he was lucky to touch bottom so he knew which way was up. He thought he was done for.
https://www.youtube.com/watch?v=jeoGEPy3WOE
https://www.youtube.com/watch?v=jeoGEPy3WOE