Barracuda attacks on humans, including completely unprovoked ones, are more common than recognized.
I learned that after an unprovoked barracuda attack amputated my left little finger and the side
of my hand in Cozumel in 2004.
I was on a dive boat with my 13-year-old daughter, Marina, and we jumped into the water to snorkel
  and see the reef. We soon saw a large barracuda, 5-6 feet long, lying on the bottom. We stayed above it
  for 10 minutes, watching it. It never moved, but was certainly aware of us. After a while we headed on to
  see more of the reef.  
My daughter had drifted about five feet away, and I started to swim toward her. At that point there
  was a stunning blow to my hand, but I never saw what hit me. I lifted my hand from the water and blood
  was pouring from it. My daughter said the barracuda had attacked. We began yelling, but the boat was a
  quarter of a mile away. Everybody was having lunch, and it took a long time before they noticed. I never
  saw my finger again, and imagine the barracuda ate it or spat it out when it turned out not to be fishy
  enough.  
Marina said that the barracuda charged me with its mouth open and that we both disappeared in a
  cloud of bubbles. She saw the barracuda charge me two more times in rapid succession, but I am certain
  that it only bit me once. Barracudas often have a stereotypical triple strike behavior. People seeing
  barracudas attack fish often see the barracuda first bite right through the middle of large fish, then
  lunge twice more to gobble down the head and tail. So perhaps the three strikes Marina saw were pure
  instinct.  
The Aftermath  
It was only when I was on the boat that I realized that my little finger and the side of my hand were
  completely gone, and the bone stuck out of raw flesh. It barely hurt, perhaps because barracuda teeth
  are so sharp that they deliver clean cuts. The crew bandaged my hand. The dive boat, having a full
  load of paying customers for the next dive, arranged for me to be taken to shore by a small boat, then
  my daughter and I had to hitchhike to town. The dive shop gave me the directions to the DAN Center
  in Cozumel. In the emergency room they injected me with a local anesthetic, pulled off the bandage,
  cleaned the wound and sterilized it with hydrogen peroxide. I spent that night in the hospital on intravenous
  antibiotics, and by dawn the next morning DAN had sent a special plane with trained staff to
  evacuate me (and my daughter) to Miami for surgery. My little finger was gone and I had come within
  about a millimeter of losing the next finger as well, but the bite had just missed the tendon so I was able
  to move all remaining fingers as normally as possible. Cuts from the outer teeth (barracudas have a couple
  rows of teeth) ran the length of that finger; had it gotten me an inch or so further over I would have
  lost all my fingers. The nerve to that finger had been severed, so I had lost all feeling in that finger, but
  it gradually recovered as the nerves regrew. I now I have a patch of brown skin with hair on my palm,
  and my thigh skin is on my forearm, so I’m built upside down! But I have full use of the hand, and most
  people don’t even notice that I’m digitally impaired.  
The barracuda that attacked me was well known to local divers, as it had frequented the same reefs
  for years menacing divers. One friend, a Cozumel dive operator, told me that he was diving with a customer
  and the same barracuda twice swam at her (she was lying still on the bottom composing macro
  shots) and butted her full force with his head, but luckily with his mouth closed, so he caused a bruise,
  not a cut.
  I later learned that the local divers actually blamed Marine Park officials for my attack. It is against
Mexican law to feed any animals in the marine parks. However, dive operators told me Marine Park
management would take important visitors out in boats and throw meat to the same barracuda that
attacked me, in order to impress their guests. This barracuda had been habituated to food handouts
from the very people supposed to prevent that from happening. But the operators all said that if they
were asked they would be forced to lie, because if they told the truth they would lose their licenses to
operate.
  Discussion    
  Every shark attack makes headline news around the world, but no barracuda attack ever does, largely
    because they are widely claimed never to happen. Mine did not even make the local newspapers in
    Cozumel.    
  I have swum with barracudas all my life (I’ve dived with tanks for 54 years). I have never been afraid
    of them, and until my own attack I had adamantly maintained that there were no known unprovoked
    barracuda attacks. I’ve also known that there could be no protection against them if they chose to attack
    you. When they go after a fish there are just a flash and a whirring noise because they move so fast you
    can’t actually see the attack, just the remnants of the fish head and tail floating afterwards and a barracuda
    gulping the center part down. What I always found amazing is that one could be swimming along
    on one side of me, then there would be a sudden flash, the barracuda would instantly vanish, and suddenly
    it was swimming on my other side. I respected them and never tried to menace or provoke them.    
  After my attack I received close to a hundred personal descriptions of unprovoked attacks and near
    attacks by barracudas. These do not include attacks on spearfishermen, who are hundreds of times
    more likely to be attacked by barracudas (or sharks) going after their catch . . . or people who spear barracudas
    and miss them or make a glancing blow, and it turns on them. One spearfisherman I know in
    Port Antonio, Jamaica, was bitten three times by barracudas in separate incidents, but he was holding a
    dying fish every time.    
  I have had many descriptions sent to me of people who were bitten around glittery necklaces,
    bracelets, finger rings, and mask reflections, or of barracudas that charged these objects and suddenly stopped just short of biting, sometimes only an inch away. However, I wore no jewelry except my
watch, which was entirely black, with the scratched glass face pointing upward. I know two people,
one a hotel employee in the Maldives, and the other a submarine engineer at the Woods Hole
Oceanographic Institution, who jumped in the water and were immediately bitten by barracudas.
Perhaps it was the splash and light reflecting off the bubbles. The engineer jumped straight back
into the boat with a tiny foot long barracuda hanging from his butt, and still has the hole to prove
it, but is reluctant to show it! Clearly, attacks are far more frequent than realized. See our presentation
at the 2005 Association of Marine Laboratories of the Caribbean Conference in Curacao:
http://globalcoral.org/BARRACUDA ATTACK.pdf
  There is no doubt that diver’s behavior can provoke attacks. An old friend from Jamaica, Phillip
    Motta, was once wiggling his fingers with his palm facing forward, and a barracuda got very excited
    and prepared to charge him until he realized it and stopped. Paul Herring was night diving in the Bay
    Islands of Honduras when a large barracuda seemed fascinated by the divemaster’s light shining on
    it, and started getting agitated, so the dive master switched it off. Immediately the barracuda attacked
    Paul’s mask face. It was unable to bite through the glass of the mask, but it knocked him unconscious
    and severed arteries in his nose and forehead. Though saved by his buddies, his face was pushed in by
    the impact and he needed many operations to recover.    
  The most horrifying barracuda attack story I’ve heard was told to me by a divemaster at the College
    of the Virgin Islands Marine Laboratory. During a DAN course on decompression chambers, the lecturers
    said there was not a single known case of an unprovoked barracuda attack, but a physician taking
    the course disagreed. He described an incident in which two divers who dived at the same location
    every week, would feed a large friendly barracuda that frequented the site. One day the barracuda was
    waiting for his handout, but they had brought no food. The first diver held his empty palms in front of
    him to indicate, “sorry big boy, no food for you today.” The barracuda bit both off both his hands. The
    second diver hugged his hands under his armpits to protect himself, and the barracuda bit and savaged
    both of his forearms so that they “looked like meat that had been through a grinder.” The doctor who
    told this story to the class ended by saying that “I was the physician who had treated them both.”.    
  Conclusions    
  What is the lesson from all of this? Don de Sylva’s advice
    40 years ago, not to swim in murky water under poor light
    conditions and not to splash around while wearing flashy
    jewelry, seems the best we can do. A diver should not automatically
    assume that barracudas never attack without provocation,
    and treat them with respect. What makes an attack so
    unpredictable is the randomness of these incidents. People
    swim with barracudas all the time and are not attacked. I am
    the only person known to have been attacked in Cozumel,
    even though the waters are full of people splashing away at
    all hours, with shiny jewelry on their fingers, necks, wrists,
    toes, ankles, navels, and other body parts, and have never
    been attacked, while I, with none of those attractions,
    was……    
  So, it’s anyone’s guess. Stay aware.    
  The author, Thomas J. Goreau, Ph.D., is the president of the Global
    Coral Reef Alliance. This is a condensed version of a comprehensive
    piece which you can find on our website at www.undercurrent.org