Growing up mostly in Florida, we always did the stingray shuffle and neither my brother or I ever stepped on one but we sure scooted a bunch of them away.
How do you know that what you scooted away weren't flounders? Flounders and stingray both bury themselves in the sand. I have waded in the surf hundreds of times. I've been stung by a stingray once. Most people never get stung. I was just one of the unlucky ones. And yes, I have fished in the surf lots of times since.
Always be careful this just happened yesterday weather pops up fast. http://www.al.com/news/mobile/index.ssf/2015/04/dauphin_island_regatta_coast_g.html#incart_river
I could usually see them swimming away. I'm sure some could have been flounder as well. Even a blind dog finds a bone, you were unlucky enough to be the bone I guess...