

Cnidaria (jelly fish and anemone’s) stings on your skin are usually nothing, if you ever want to swim with jellyfish a pair of nylons over your exposed body is generally enough to keep you from being stung, but whatever you do don’t put your tongue (or other sensitive parts) to one, because the toxin can totally mess you up (generally just major swelling like the worst bee sting you’ve ever seen, but swelling could always lead to other problems). Some of the most potent venoms on the planet are secreted by the cnidaria, but there will definitely be (actual) signs warning you away from areas they exist in.
Not enough soap in your pool.