We had a lovely vacation on the Gulf in Florida last summer...until an afternoon storm stirred up a bazillion jellyfish. All of my kids got stung in multiple places, and that was wading in the surf barely waist-deep. One little girl near us apparently had a serious reaction because they had to send in a Life Flight helicopter to rush her to the hospital. (I didn't hear if it was from jellyfish stings or a stingray, which were also swimming in the surf only a few feet from shore.) One of my sons still has visible scars from the welts.
Also, a few miles over from where we were splashing about is where one kid was bitten by a shark this summer, again in waist-deep water.
It doesn't hurt to remind people to be careful.