Dive Atlas of the World. Jack Jackson

      DRIFT-DIVING

      Drift-diving can vary from pleasantly drifting in a gentle current to high-voltage rushes as divers are swept along walls and gullies. The main concerns are good boat cover and becoming separated from div ing buddies. Divers not using surface marker buoys should carry a delayed deployment surface marker buoy or, better still, a high-visibility rescue tube or collapsible flag, which can be raised above the swell.

      Insist that the chase-boat crew follow the surface marker buoy or divers’ bubbles and do not go to sleep or have loud music preventing them from hearing divers’ whistles when they surface. Power whistles are better at attracting the boat cover than manual whistles and an old CD can be used as a heliograph.

      Buddies, and preferably the whole group, should enter the water together so that they do not get separated on the surface and they should try to keep together underwater. If divers do get separated from their boat cover, it is wise to tie a buddy line between each other, inflate the BCDs (Buoyancy Compensator Device) and conserve air. It is usually best to retain weight belts unless buoyancy is a problem; in certain circumstances it may be better to jettison the scuba cylinders.

      Divers wanting to fin ashore while wearing a normal BCD rather than wings will find it less tiring to fin on their backs. At the shore, untie the buddy line before trying to swim through surf or breakers.

      WRECK AND CAVE DIVING

      When diving in enclosed overhead environments, it is not easy to reach the surface in the event of equipment failure.

      Any level of diver can enjoy diving around a wreck, but penetrating large wrecks is advanced diving and novice divers should only attempt it when accompanied by an instructor. Plan dives to coincide with slack water and wear gloves for protection from sharp metal. Carry a sharp knife and a suitable monofilament line cutter or shears for cutting fishing line and nets. Have a good dive-light and carry another as backup. Make sure that equipment is streamlined against the diver’s body where it cannot snag.

      Divers should tie off a guideline before penetration and feed it out as they go, tie back any doors or hatches, so that they cannot close in a current. Remember that exhaust bubbles disturb sediment, as do fins and hands. Leave plenty of air to get out of the wreck and back to the surface.

      Cavern diving, where divers are always within sight of daylight, is not difficult. However, cave diving, beyond any source of daylight, requires a safety guideline so that the divers can find their way back to safety in zero visibility. They will also need separate backup sources of light and breathing-gasses. Most important is the rule of thirds: divers turn around when one-third of their breathing gas is used up, leaving one-third to find their way out and one-third for emergencies. Apart from exhaust bubbles disturbing sediment as they strike the roof, divers can minimize the disturbance of sediment by learning to use gentle, shallow fin-strokes.

      NIGHT DIVING

      For night dives, divers should choose a shallow dive with easy marks for navigation, with which they have already familiarized themselves in daylight. The easiest night dives are along reef edges, where divers can swim out along the face at one depth and return along it at a shallower depth. If there is a current divers should set out against it and return with it.

      Avoid dive lights that are too powerful and carry a spare as a backup, but spend some time with your lights switched off. When your eyes are accustomed to the dark, wave your arms about and you will notice phosphorescent plankton and, in caves, you may spot the bioluminescence of flashlightfish.

      TEMPERATE WATERS VERSUS TROPICAL WATERS

      Many divers do most of their diving in temperate waters. Shipwrecks are the most popular sites, but the marine life can be just as interesting as in tropical waters. 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.