The Bobbit Worm.

This animal is straight out of a horror movie. I’m convinced they came from the meteor that wiped out the dinosaurs. The worm falls into the category of ambush hunters, and they’re pretty good at it. They patiently wait for a small fish or crustacean to move across their hole, then grab them with their mandibles. After they have a secured grip, the prey is pulled beneath the ocean floor.

Over 95% of their body is hidden in the ground. They only appear to be a few inches, but they can be up to 10 feet long.

Well, these 3:

The Chironex fleckeri, or more commonly known as the Australian box jelly. These nasty little things are quite often lethal and very hard to see in murky waters. There are a few other species of box jelly that are deadly too but even the non- lethal ones sometimes claim lives because of people being allergic to them. Nasty nasty critters.
And then we have this little asshole:

This is the cookiecutter shark. It won’t kill you; it will just take a bite. It got its name from the round bite wounds it causes. If you’ve ever encountered dolphins or sharks in the warm waters in Hawaii, you might have noticed that many of them have round scars where bites were taken out of them. People as well, by the way.

And the cookiecutter is not very particular in what it will bite either: small fish, other sharks, dolphins, whales, humans, fishing nets etc. If has even been credited for damaging submarines and its equipment by biting and damaging its electric cables and sensor equipment. Not even oceanic telecom cables are safe from them.
Absolutely terrifying monsters.
Lastly, the Irukandji jellyfish. They are even smaller than the box jelly, similarly deadly and as you are often not aware of what you are stung by, treatment is often too late.

The the yearly cost for medical treatment is estimated at about 3 billion Australian dollars. The only good thing about them is that they are only found in a relatively small area on the northern coast of Australia.
Lampreys

Humboldt Squid

Angler fish
