22. Box Jellyfish
There are a lot of jellyfish in the sea. Telling them apart is not easy, but recognizing this particular jellyfish is worth it. Some, not all, produce a venom so poisonous it can kill humans if not cause them massive pain. The best way to avoid them is not to swim in water where someone has spotted one, which is typically in the Indo-Pacific region. That said, they can range from tropical to subtropical regions. A quick scan of the surface and then again from in the water is usually enough to stay safe.