It's difficult to describe the behavior of sharks since the species vary so much. There is also individual difference, much like humans. I've had friends who passed up a big lobster because a more or less harmless nurse shark was sleeping under the same ledge. On the other hand, others have gone on kooky shark feeding expeditions without a care.
The sharks that a diver is most likely to see in the Keys and general area would be nurse, grey reef, white tip reef, black tip reef, lemon, sand tiger, bull, tiger, dusky, silky. Less often would be the blue water sharks, mako and white. Of these, the white is often aggressive but their specific actions depend on their size and what they normally hunt in that area. Diving near the US and Mexico West Coast islands clad in a black suit is fairly dangerous. Most whites attack from below but also do occasionally cruise the surface. California divers looking for a good site may spot them on the surface and change their mind. Even so, this might only happen once in fifty trips. One strange experience, in 1968, near Marathon,FL, I saw one resting on the bottom. It followed me back to the boat. It was nearly as long as my 18 foot boat.
A good place to see reef sharks would be Bimini or the Berry Islands. The Bahamas are infested like cockroaches. Many SCUBA divers who take the big headboats to popular deep(100')reefs apparently don't see many, (otherwise I would expect to hear about it). However, spearfishermen, freedivers, who travel in small parties and frequent shallow reefs will see several every day, especially if there are also string fishermen on board.
The most common shark sighting is the cruising grey reef shark. Mostly, the diver will see them near the bottom or midwater, not on the surface as in the movies. They will often come within 20 feet or so and make a turn or a circle. Unless you have a fish, charging the grey shark with a pole or gun barrel will scare it off. Usually, this is not necessary as it will maintain distance. When spearfishing, the shark will show up the instant a fish is speared or a minute or two later. If you are carrying a fish in a bag and the shark cannot see it, it will sometimes woosh by or even slam into you and depart temporarily. Then it will come back and be very difficult to drive off. Others will likely appear. If it can see the fish in plain view the shark will attempt to snatch it. These kind of things are more likely to occur if you are not looking at the shark, eg do not see it coming. The stare makes many sharks nervous and they will back off. Sharks can see as well as we can. Better, perhaps, as their peripheral vision is good and night vision is phenomenal. Like most fish, they see details better when swimming broadside as opposed to coming head on. In clear water, sharks do not attack waving hands unless there is a fish in it. They know the difference. I have never attended a shark feeding session; the kind where, as described in the previous posts, somebody got bitten accidently. I can believe it though.
The bulky and sluggish looking bull shark, like the others, does not act the same way each time. However, they will almost always move in if a fish is speared. Small ones will circle cautiously, a big bull shark may charge in explosively, take a quick look and depart. Sometimes they stay to harrass the diver. Bull sharks are not as common as the grey, so this does not happen every day. When it does, the spearo may have to drop his fish and defend himself. This becomes a game where the bullshark will try to weave, dodge and get past the outstretched weapon. Striking the shark with the gun barrel will not cause a sudden attack but the shark will not leave either. It will act determined and stubborn. If you shoot the shark it will turn and flee while breaking the string or taking the whole gun. Where it concerns the non spearfisherman, a bull shark will sometimes swim right by, sometimes very close, and not seem to see you. However, although their eyes are small, the shark can see you. Often, the big ones have several pilotfish. They sometimes swim in shallow water, the surf, brackish bays, and over reefs near a dropoff. They seem dangerous to swimmers when bait fish are inshore.
The bigger the shark, the more bold(except for the sand tiger and nurse); a prime example is the tiger shark. These are dangerous to spearfishermen, particularly freedivers. They seem particularly aggressive off the Hawaian Islands and probably responsible for many "lost" swimmers. In the Bahamas and points south they are far more common in shallow water than one would imagine. Fly a small plane around the area and you will get an eyefull, some up to 18 feet long and in 10 feet of water. Even so, to the diver they seem far less common than the greys, etc.
A pack of sharks is usually more bold than a single animal. This is particularly true of grey sharks. Fortunately for the spearo, these sharks prefer to stay together rather than circle and harry the diver individually and from different angles. Thus, if the diver fends off the first brush and backpedals toward the boat, the pursuing sharks do not try to outflank the diver. They pursue in rough formation making them easier to keep account of.
There is no way to describe sharks behavior in a few paragraphs. Shark behavior is complicated by territoriality, the fullness of moon and who knows what. They are reported frequently, and in a bad light, because there are more people in the water and because fishermen have depleted the normal food supply. Attacks linked to feeding behavior are often "accidental". The surfing shark has no choice but to strike at anything that moves. He will go hungry otherwise as he can't see well in the murky water, bait fish are fast, and there is no time to deliberate.
As far as I can tell, sharks do not care what colors or jewelry the diver is wearing. The barracuda, however, is attracted by bright objects. Whites and tigers are programmed to attack certain dark shapes on the surface. Free divers and surfers are generally aware of this.
As a new diver, it is natural to be apprehensive about sharks. The best cure for that is to gain experience in swimming in sharky water. It is better to do this in company of other divers to build confidence. Unless you are a spearfisherman, the chances of having a hair raising experience are fairly slim. Soon, with experience, you will tend to view their behavior as similar to and unpredictable as dogs, and learn to watch their body language. Just ask the postman about that.
