The real hazard of psychological stress lies in its manifestation with the Scuba diver. As stress rises, the Scuba diver may experience anxiety, become distracted and suffer impaired function. He may suffer decreased awareness, called perceptual narrowing, which causes him to overlook things that would normally be obvious. This can set up the very thing the Scuba diver fears; for example, the third Scuba diver in the previous example, experiencing perceptual narrowing, fails to notice a worn fin strap. After entering the water, the strap breaks and he loses the fin as he struggles to reach the swim line; without the fin, he misses the line and gets carried away by the current. With his fear realized, his likely psychological response is to trigger more fears, raising his anxiety still farther, probably to the panic point unless something or someone intervenes. 

Psychological stress also sets off physiological responses in the Scuba diver's body, which may be physical stressors themselves. The Scuba diver may experience nausea, vomiting, diarrhea or a need to urinate. The person may become tense and have muscular trem­ors or a headache. Adrenaline may accelerate the heart, or cause an irregular heart beat and chest pains. Breathing accelerates, too. If the Scuba diver's underwater, the breathing rate can exceed what the regulator can deliver, creating a sensation and suffocation. At the surface, the Scuba diver may hyperventilate and have trouble getting air through his snorkel or in splashing waves. This makes anxiety rise further, triggering even more breathing and a greater sensation that the Scuba diver can't get enough air.