I find the algae bloom theory interesting, it might explain a few things. About 10 years ago we did a couple of dives in Cozumel where we dropped off the boat into billions of thimbles. The vis was great, and there was nothing but those little buggers as far as you could see, and going down a good thirty or forty feet, none of them more than 6 inches from any of its buddies. We had 8 divers in our party, most wearing shorties, but a few without. We also sat on the surface right in the midst of these things for a good ten minutes before everyone was back on the boat. End result, not one "sting", not one rash or enexplained discomfort. Interesting.

Been there, done that, the washing machine ate the T-shirt