What is the most terrifying marine animal?

 This creature would stop me from wanting to swim! Fortunately, she only lives near the coast of Australia.

Let me introduce you:

Medusa irukandji (or Carukia barnesi)

Yes, the tiny creature in the little tube...
Lousy, right?
Don't be fooled by appearances!

It is a cubomedusa (1), very small (3cm or much less). But whose almost invisible tentacles can reach 1 meter!
Which makes it formidable for swimmers, who most often don't see it at all!
Its venom is responsible for a syndrome (Irukandji syndrome) which causes anxiety, nausea, vomiting, pain in all limbs and breathing problems. its venom can also interrupt electrical impulses that are sent through the heart, between 20 and 40 minutes after the sting.
This jellyfish is found on the northern and eastern coasts of Australia, from late October to early May. Be careful, unlike other jellyfish, it is not confined to coastal waters but is also found on the high seas.

(1) a cubomedusa like the famous one Box jellyfish or cubozoa Chironex fleckeri

The only "advantage" of which compared to the previous one is that it is of a more visible size because it is larger. Transparent blue, its tentacles can reach 3 meters long!
Otherwise it is one of the most toxic creatures in the world.

Upon contact with it, the toxic substances it administers can cause a man's death within 5 minutes. In case of contact, prevent the victim from drowning, sprinkle the tentacles with vinegar before removing them. The best thing next is to call for help quickly.
It is also referred to as "Sea Wasp" or "Sea Stinger". In 1980, in the Indo-Pacific region, there were 55 deaths due to painful bites from gigantic tentacles.
She also lives in Australia, a coastal area.

What are some amazing facts about the ocean and marine creatures?

 

• Only ~5% of the ocean has been explored.
• The immortal jellyfish, as its name suggests, is biologically immortal meaning that it can potentially live forever.

• Deep sea vents can reach over 700 degrees Fahrenheit.

• Across the world, there are an estimated 3 million shipwrecks in the ocean.

• Around 97% of the Earth’s water can be found in the five oceans.

• Just one millilitre of coastal water taken from the ocean's surface can contain up to 10 million viruses.

• There are more viruses in the ocean than there are stars in the Milky Way.

• The box jellyfish is one of the most venomous marine creatures in the world, with its poison being able to cause paralysis, cardiac arrest, and even death in just a few minutes.

• The deepest part of the ocean lies in the Mariana Trench, called Challenger Deep. It has been measured to be 10,984m (36,037 ft) in depth. In comparison, Mount Everest is only 8,849m in height.

• It is completely pitch black at the bottom of the ocean, leaving it devoid of any plants as photosynthesis cannot occur without sunlight. The only animals which are able to live down there are ones that have adapted to the absence of light and crushing pressures.