Deep-sea diving is primarily performed by professionals who are working underwater, such as welding oil rigging, or for scientific exploration. Recreational scuba diving is swimming to a depth up to 130 feet, and deep-sea diving is usually 130 feet down to 250 feet below the surface, when compressed air stops working effectively. Marine animals are a small but real threat to deep-sea divers, who typically have matters of bigger concern such as pressure, breathing and decompression issues.
Sharks don't feed on people typically, but as opportunistic feeders, they won't turn away an easy meal. They are often attracted to the scent of blood in the water or chemicals released by fish in distress, and they might sample a taste of a diver swimming near those scents. A deep-sea shark attack is rare, but because you must usually ascend slowly when diving deep, an attack can cause traumatic blood loss before you reach the safety of the surface.
Even thick wetsuits can't protect you from every stinging sea creature. You might happen upon some, such as jellyfish, and be unable to avoid their long tentacles. Others, like stonefish or venomous cone shells, you might brush against or pick up by accident. Most of these stings are painful but not deadly, especially when your suit or gloves take up part of the sting. However, some stings, such as a stingray or stonefish, can cause local paralysis and weakness that can affect your ascension to safety.
Sharks aren't the only sea creatures who bite. Fish nibbling at your toes are unlikely to cause any problems, but there are sea snakes and octopi that can. Sea snakes can be extremely poisonous, but they often can't inject poison into people because of the way their teeth are shaped. Even when no poison is injected, the bites can be deep and painful. Some octopus species, such as blue-ringed octopus, can also inject venom when they bite you with their center beak. The venom can paralyze you in minutes, but your wet suit might offer some protection.
Dangerous bites and stings from marine animals happen, but only rarely. A bigger danger for deep-sea divers is from equipment malfunction, since they rely completely on a breathing apparatus to provide air. General dangers from swimming under deep water pressure are also major concerns. Descending too rapidly can cause barotrauma, or damage to the inner ear, and ascending too quickly can lead to decompression sickness or a pulmonary embolism, both of which can be deadly. Divers who stay under the intense pressure of deep-sea water can suffer from nitrogen narcosis or oxygen toxicity. These conditions often lead to confusion and trouble seeing, but they can also be fatal.