In general the visibility and surface conditions will not be that good. By contrast, when diving in the warm, clear water of the tropics, surrounded by colourful marine life, divers are likely to be more relaxed. The main danger with such clear water is that you are likely to dive deep without realizing it. Many of the more popular diving holiday destinations are in areas where tides and currents are minimal and sea conditions usually calm.
ICE DIVING
Always be prepared for equipment failure – even weightbelt buckles have failed. Although the water temperature cannot fall below –1.8°C (28.7°F), or it would be frozen solid, wind-chill can make air temperature many degrees colder. A full-face mask makes it difficult to access a backup regulator. Each diver should have two separate regulators, either on separate scuba cylinders or on a single one with a V-manifold. Cylinders should be filled with air that is as dry as possible. Cylinders and regulators should be stored out of the wind in a dry place until entering the water. Regulator first stages should be environmentally sealed against the ingress of water and not breathed through until both first and second stages are submerged to avoid condensation freezing the regulator. Divers should each be attached to a line strong enough for hauling them to the surface. Each line should be tied off securely and attended by someone doing nothing else, feeling for an agreed series of rope signals from the diver. Erratic pulls, unreadable pulls or no response, should be treated as an emergency and the diver pulled up.
Diving under ice is a surreal experience. The ice forms amazing shapes and at high latitudes the animals exhibit gigantism.
It is wise to wear thin exposure suits against creatures that sting, but the best chance of ruining your holiday comes from sunburn or insect bites when you are not diving.
ARCHAEOLOGICAL DIVING
Archaeological diving is usually restricted to academics but there are times when they are grateful for amateur help and some of the finds in the Egyptian Mediterranean are now open to guided diving tours. Where such sites are close to a port there will be sewage and industrial pollution in the water and oil on the surface. Divers should take a course of broad-spectrum antibiotic as a prophylactic against intestinal infections. Diesel oil on the surface causes skin-burns and degrades exposure-suit materials, so wash all equipment (and yourself) with freshwater immediately after immersion.
UNDERWATER VISIBILITY
In mid-oceanic waters the vertical visibility can reach 100m (328ft), but horizontal visibility greater than 50m (165ft) is mythical. Coastal waters are affected by rain, run-off, disturbed bottom sediment, agricultural, industrial and domestic pollution, landfill, quarrying, volcanic eruptions and plankton blooms, so the visibility is less. Water clarity is better over deep water or a solid bottom. Ebb tides lower water clarity by carrying sediment off beaches and reefs; visibility usually improves on a flood tide. Care with buoyancy will prevent divers from disturbing the bottom sediment.
Heavy rain and wind reduce visibility if bad weather causes freshwater and saltwater to mix or if it sets off a plankton bloom. Offshore waters appear blue, but the decaying organic matter in coastal waters is yellow, so some of the blue is filtered out and the waters look green. Local mineral deposits or mining are also factors that can affect the colour of the water.
REPETITIVE DIVES
For surface intervals greater than 16 hours, divers can assume that there is no excess nitrogen remaining and can therefore treat the next dive as if it were the first. A second dive in less than a 16-hour period must be classed as a repetitive dive. The possible depths and times can be calculated from a dive planner, dive tables or shown by a diving computer. Divers performing repetitive dives over several days should take a complete day off after four days to allow the nitrogen remaining in the body tissues to dissipate completely.
Wrecks are perfect sheltering places for shoals of tiny fry and larger juvenile fish. This is the wreck of the Nebo at Aliwal Shoal.
DECOMPRESSION DIVES
Decompression dives are not recommended for recreational divers and most American recreational dive planners do not allow for them, although European dive tables do. There may be times when, for whatever reason, divers exceed the no-stop dive time limit at a given depth and then have to make stops on the ascent, long enough to let excess nitrogen diffuse out of their body tissues (decompression stops).
Different training agencies recommend different depths and times for these stops, though the deeper ones are more easily maintained in a swell. If the divers have not been very deep and not for too long, then one stop will be sufficient, usually at a depth of between 3m (10ft) and 6m (20ft). If the divers have been relatively deep or exceeded the no-stop time for longer, they will have to make additional stops at greater depths and then a longer one between 3m (10ft) and 6m (20ft). Special tables are available for diving at altitude or on Enriched Air Nitrox.
FINISHING A DIVE
Divers should finish all dives, whether decompression or not, with a five-minute safety stop at 3–6m (10–20ft). It can be difficult to hold a stop at 3m (10ft) in a swell. It is easier to hold 5m (16ft), which allows leeway if the swell causes you to ascend a little.
ALTITUDE AND FLYING AFTER DIVING
When diving at altitude, divers must use tables or computers designed for altitude diving. The reduced pressure in aeroplanes at height can cause large bubbles to form, causing decompression sickness in divers who fly before their body has had enough time to release most of the accumulated nitrogen. Even worse, high-flying aircraft cabins have been known to depressurize in flight. Divers intending to fly should allow at least 24 hours after diving.
CLIMATE CHANGE
Scientists disagree over the rate and likely extent of change resulting from global warming. Similar events have occurred throughout history and some glaciers are currently expanding. In Antarctica the Ross Ice Shelf has grown several kilometres (miles) in the last two decades. However, most glaciers are receding and large amounts of polar ice are melting. The resulting increase in sea levels threatens the existence of low-lying islands such as the Maldives and increases the risk of flooding on the lower sides of regions such as the UK.
THE EL NIÑO-SOUTHERN OSCILLATION PHENOMENON (ENSO) AND LA NIÑA
El Niño conditions can result in strange weather patterns in some diving areas. Warm water means that many sharks descend to deeper, colder water, but most importantly, animals that have symbiotic algae may expel them as in coral bleaching.
In normal years, the upwelling cold water in the trade-wind belts off the west coast of South America leads to rich fishing and causes the overlying air to cool below the temperature at which water vapour condenses (dew point), producing fog. However, sea-surface temperature changes in the equatorial Pacific sometimes produce a major climatic disturbance known as El Niño, Spanish for The Boy Child, because Peruvian fishermen noticed that it often began around Christmas. During an El Niño/Southern Oscillation Phenomenon, a weakening of the easterly trade winds in the Central Pacific means that warm surface water is no longer pushed west to allow for a cold, nutrient-rich upwelling off the coasts of Peru and Ecuador. The warmer sea-surface temperature
