Sharks are probably the most dangerous fish for humans, but we have written about them in another article ("The Most dangerous Sharks").
Apart from sharks, many potentially dangerous fish are venomous, and they too have been treated in another article ("The Most Venomous Fish").
Those two categories excluded, there are surprisingly few fish species that can be regarded as dangerous to humans... yet enough still to justify an article!
(Note that the different species in this article are by no means ranked according to their dangerousness).
1. Barracuda (family Sphyraenidae, order Perciformes)
There are some 20 species of predatory fish known as barracudas. They usually occur in warm tropical areas, though some are found in more temperate bodies of water.
Some barracudas are rather small; others, notably the great barracuda (Sphyraena barracuda) of the Atlantic, Caribbean, and the Pacific, can measure as much as 1.2 to 1.8 meters (4 to 6 feet).
Barracudas have small scales. They are swift and powerful, with a slender form, two well-separated dorsal fins, a jutting lower jaw, and a large mouth with many sharp large teeth.
These predatory species are primarily fish eaters of smaller fish, such as mullets, anchovies, and grunts.
Barracudas are bold and inquisitive, and fearsome fishes, that can be dangerous to humans. The great barracuda is known to sometimes attack swimmers. It can cause harm with its sharp jagged teeth and strong tearing jaws; slashing and creating jagged tears in your skin. The great barracuda is quite common in Hawaii, notably, where it is encountered in open waters as well as in bay areas in the shadows, under floating objects.
Barracudas are attracted to shiny, reflective objects , that look like the scales of a fish (in other words, like a potential prey). The best protection against barracuda is not to wear any jewelry and other shining objects.
If you (or someone else) get bitten, get out of the water, stop any bleeding and treat for shock by keeping yourself or the victim calm and warm. Seek medical treatment.
2. Grouper (subfamily Epinephelinae, family Serranidae, order Perciformes)
Groupers belong to a number of different genera, but the common name grouper is usually given to fish in one of two large genera: Epinephelus and Mycteroperca.
The Nassau grouper is common resident in the waters off the Gulf of Mexico and the Caribbean. Some divers have been "bitten" by over friendly Nassau groupers that are used to human interaction in popular dive feeding sites. During feedings groupers occasionally will take the entire fist and forearm of unsuspecting diver into their large mouths.
Grouper have several sets of teeth, placed in the mouth to act as raspers or holding teeth. The fish gulps down its prey using its teeth to prevent the fish from escaping. Though the teeth are not used to tear or slash (as with barracuda or sharks), the problem comes from the fact that some groupers can weigh as much as 800 pounds! Most bites on humans primarily result in loss of skin from the back of the hand and fingers, often followed by a severe infection.
3. Sea Bass (family Serranidae)
The sea bass is essentially found in open water. Like its "cousins", the groupers, it is potentially dangerous because of its large size. Indeed, it can remove large pieces of flesh from a human.
4. Moray Eels (family Muraenidae)
There are approximately 200 species of moray eels in 15 genera. Their average length is 1.5 m (5 ft), but the slender giant moray (Strophidon sathete) can measure up to 4 m (13 ft). The largest species, in terms of total body mass, the Giant moray (Gymnothorax javanicus) reaches almost 3 m (10 ft) for a weight of over 36 kg (80 lb).
Like barracudas, moray eels have been known to attack humans and inflict vicious bites. One should be careful with those species when near reefs and in shallow water.
Yet, moray eels are not, by nature, aggressive towards people, but can attack when provoked. A number of divers have been bitten by moray eels, sometimes severely. Indeed, moray eels have sharp teeth designed to lock on to prey.
People are quick to blame an animal and to label it "aggressive", but in the case of moray eels (as with potato cods, wrasses, groupers, and other fish), many attacks are an indirect result of the foolish practice of fish feeding by hand. Fish that are used to receiving handouts can approach divers on sight and "accidentally" bite a hand which they believe to be holding food.
5. Surgeonfish or Surgeon fish, Doctorfish, Unicornfish, Tang (family Acanthuridae)
These tropical species, thus, are not dangerous because of their size, but because at the base of the tails of the aptly named surgeonfish are razor sharp blades or needle-like spines which can inflict nasty cuts. No venom, however, is involved.
6. Piranha (genus Serrasalmus, family Characidae)
Piranhas live in the tropics and are restricted to the Amazon bassin in northern South America. They are the only significantly dangerous found in fresh water. However, only 4 of the 20 species of the genus Serrasalmus are considered dangerous.
These fish are fairly small, about 20 to 40 centimeters (roughly 1 foot), but they have very large teeth and travel in large schools. They are, especially attracted to blood, and can devour a 135-kilogram hog in minutes.
Piranhas appear to be most dangerous in the low-water season, when hungry fish become concentrated in pools.
Though piranhas do attack humans, they are not man-eaters folklore would suggest. Most people are bitten when the fih is removed from the water when fishing, and victims are much more likely to lose a toe than to be killed and eaten. It is now believed that in many human deaths attributed to piranhas, the predators were actually only scavenging on drowned or otherwise already dead persons.
Introduction to Dive Medicine Dangerous Creatures of the Sea