Barracuda
This is not the kind that lurks around your pool looking for leaves. This is the serious-looking and seriously-scary sea scavenger. Many times, the barracuda has mistaken swimmers and divers as prey. On other occasions it has believed humans were big predators and even waited around to eat scraps of their meals. I doubt that ever happened.
Blue-Ringed Octopus
Once again, a beautiful seabed creature that could kill. Not only does it have enough poison to kill a human-being, but it can kill 30 of us! It holds the equivalent of 10,000 times the amount of potency that cyanide does. Most health insurance coverage probably doesn’t cover something like this.