2. Cone Snail
You read that right, a snail. The Cone Snail has a harpoon that it uses to inject you with its venom, of which one drop is enough to kill 20 humans. What makes things worse is that there is no anti venom. On bigger cone snails the harpoon has been known to penetrate the skin through gloves or wet suits. They are primarily found in the Pacific Ocean and Indian Ocean.
Luckily this is a sea snail so as long as you stay out of the water you don’t have to worry about anything, but if you do feel brave enough to go swimming make sure you don’t pick one of these bad boys up. That is now one more sea creature to worry about. At this point why even bother going in the water.