Environmental hazards, as DAN and the cave diving bodies describe them.

Currents

A DAN Alert Diver article by Mike Ange, Diving in Currents (1 November 2010), calls currents one of the most underestimated and physically demanding conditions a diver can face, including for experienced divers. Ange explains that water is so much denser than air that even a moderate current pushes harder than a storm-force wind. Currents can raise gas consumption, wear a diver out and make it impossible to get back to the boat.

The article gives a sense of scale:

DAN adds that current changes with depth. Flow at the surface and in midwater is generally stronger than near the bottom and may run in a different direction, and tidal currents can reverse during a dive. DAN links swimming against a current to higher gas use and faster uptake of gas by the body, which it associates with the risk of decompression illness, and it says divers should get training before diving in strong current.

Separation from the boat

A DAN Alert Diver article, The Boat Left Without You: Now What? (author and date not recorded in our research), says being left behind by a dive boat is rare, but being separated from it is far more common. It names the contributing factors as low visibility, currents, rough seas that hide divers from the crew, hypothermia even in fairly warm water, and careless tracking of who is aboard.

The article says dive operators should use credible ways of accounting for divers going into and out of the water, citing DAN's own Diver IDentification System, with a second person confirming that everyone is aboard before the boat leaves. It also lists the kinds of surface signalling devices divers carry: surface marker buoys, signal mirrors, rescue streamers, whistles and air horns, and dive lights and strobes. DAN recommends carrying a mix rather than relying on one. How to use them is taught in training; the devices themselves are described on lights and signalling.

Cold, and heat

DAN's Travelers Medical Guide defines hypothermia as a core body temperature below 35 C (95 F). It says water draws heat away from the body far faster than air, which makes immersion a major cause. Its other points:

• Sudden immersion in cold water, below about 15 C, without thermal protection can set off a dangerous cold-shock response, including a gasp reflex that can lead to inhaling water.
• Heat loss continues, more slowly, even with protective clothing, and even warm tropical water of about 29 to 33 C can cool a person slowly without it.
• How fast someone cools depends on water and air temperature, wind, tides and currents, body build (smaller and leaner people cool faster), what they are wearing, and their age and health.

Cold also turns up in DAN's lists for other conditions: it is a known risk factor for decompression sickness, and a contributor to immersion pulmonary oedema (see other diving illnesses). Overheating works the other way round. DAN says divers rarely meet water warm enough to overheat them, so hyperthermia mostly happens at the surface or on land, for example in insulated suits, particularly dry suits, or while carrying gear in hot weather. Hypothermia and hyperthermia both need prompt medical care from a doctor.

Marine life

A DAN Alert Diver column by Bill Ziefle, Hazardous Marine Life (22 November 2024), says encounters with potentially aggressive marine animals are almost always uneventful. The risk cannot be removed entirely, but Ziefle writes that close attention to one's own behaviour and the animal's generally prevents harm.

DAN's health resource on marine life injuries, by Paul S. Auerbach, MD, says most injuries come from chance encounters, and that scrapes from coral are among the most common marine life injuries to divers and snorkellers. It describes stingray injuries as defensive, mostly to the feet and legs of people who step on rays hidden in sand, and jellyfish and fire coral stings as the result of contact with tentacles or other parts. Its prevention principles are to stay alert, respect animals' personal space and handle animals as little as possible, which DAN says is better for both the animal and the diver.

The more serious stings, such as Irukandji syndrome from certain box jellyfish, are described on other diving illnesses. Stings and other marine life injuries are medical matters that need prompt care from a doctor.

Overhead environments: caverns, caves and wrecks

Where there is no direct way to the surface, bodies such as the NSS Cave Diving Section (NSS-CDS) teach diving as a separate discipline. On an undated course page, NSS-CDS describes its Cavern Diver course as a safety and awareness course for open-water divers aged 18 and over. It states that the course is not a prerequisite for any level of cave diver certification and does not count toward one. NSS-CDS says the course aims to help divers avoid the mistakes that commonly lead to cavern and cave deaths, and it stresses keeping within sight of the entrance and daylight.

DAN lists two peer-reviewed studies of cave diving deaths among its fatality research: Potts et al. (2016), covering thirty years of American cave diving deaths, and Zeigler et al. (2008), comparing cave diving deaths before and after 1980. Their findings were not read for this page.

Snorkellers too

DAN's Travelers Medical Guide lists strong tides or currents and being struck by watercraft among the causes of snorkeller deaths, alongside sudden cardiac arrest and immersion pulmonary oedema.