The answer is depending on what you mean by deadliest.
there are many undiscovered species of fish in our ocean so it would be hard to say.
Here are some different fish that are known for being deadly:
barracuda- not an incredibly big fish, but very fast, one of the fastest in the ocean. they live in shoals and have very sharp teeth. they have attacked people before, and their attack is very sudden and quick.
shark- there are many different sharks in the ocean some harmless like the basking shark, others like the bull shark are more aggressive. white sharks are mostly thought to be the most dangerous but that is not necessarily fact. sharks bite through curiosity to see what an object is, and they are not particularly interested in humans although they will attack.
marlins- often known as sword fish, these are very fast, and have very striking colours, they do not attack humans but are deadly predators for their prey (fish).
There are thousands of others but these are just a couple, you can also get salt water crocodiles if that falls into your category.
The venom of the stonefish is the most toxic of all the fishes in the animal kingdom. The two members of this particular family, Synanceja verrucosa and Syanaceja horrida, live on reefs and appear to be "part of the scenery" as they are well camouflaged. Their venom is stored in sacks at the base of their dorsal spines, and the spines act as "needles" to inject the venom should someone step on one of the fish. A link to the Wikipedia article (which has some pictures) is provided.
Puffer Fish. Hands down. Second most poisonous creature on the planet.
Parannas are pretty mean or a shark if you would count that as a fish
Fish aren't really dangerous, unless you're talkin about a paranha. But the most dangerous PREDATOR in the ocean is a killer Whale, or Orca. (both really mean the same thing!)
peacock lion fish
The pufferfish.
a shark
It like not poisonous but the most venomous fish known on earth is the reef stonefish
Jellyfishes are known as most dangerous, if a jellyfish bites you then pour vinegar on the wound and call an ambulance
piranha
You must pick up the shark and smack the pygmy into the water and let him drown. A lot of answers say to get the sword fish but this is false
only if they hit you:)
The cells of the saltwater fish would gain too much water.
Jellyfishes are known as most dangerous, if a jellyfish bites you then pour vinegar on the wound and call an ambulance
fish
piranha
In my opinion it isthe bull shark. There are many species of dangerous fish in all area's of the world.
probably surströmming
jelly fishes
The most dangerous fish in the world is the great white shark. it has very sharp teeth to grind its food. It its so much and never gets stuffed.
The Great White Shark would be the more dangerous of the two.
Piranhas are freshwater and quite dangerous.
they are dangerous to the ecosytem
The blue-bottle jellyfish and the box jelly-fish. keep your distance.
fish in the artic lowlands are one of the most dangerous fish in the world. they originated form San Lorenzo Guatemala. They Hurt children and will hurt you!