I'll add jellyfish to the list. Wait, no, because technically I didn't SEE it.
I just felt an amazing amount of pain coming from my foot suddenly. I have hardware in that area from a leg break and I seriously thought I had somehow broken my foot due to that, even though all I was doing was stepping on the board to beach start after talking to someone on the shore. Kept flexing it and seeing if it caused more pain to put weight on it, it didn't. Then I saw the rash forming and realized what it was.
Intense electrical/hot pain that probably felt worse than breaking my leg at first. Only thing that felt more powerful with pain was a kidney stone, where I actually blacked out.
Apparently in the mediterranean they have a few types, I think this one was called a mauve stinger due to the symptoms they report for that. Too bad I didn't see it because they look kind of pretty.