The Night Dive; its a different style of diving to day diving, some divers either love it or they hate it. To those who have never been on a night dive, the obvious question is 'why bother?' The answer lies in the fact that many underwater animals, which hide during the day, come out to feed at night and can be easily spotted in a diver's torch beam. Many of the species of crustaceans and molluscs use the night hours in this way, and some of their predators, such as conger eels, follow suit. By day, many of these species can only be seen at the back of holes or under rocks, but by night they can be found in open spaces.
Although visibility under the water is obviously restricted by the darkness, this situation should not be confused with low-visibility situations caused by particles suspended in the water. At night, the clarity of the water is often very good, and this sensation is heightened owing to the fact that the diver's horizon is restricted to the penetration of his torch beam. This causes the diver to concentrate in the objects that his beam of light shines upon. Sites dived at night often seem to have a lot more life than when dived during the day. In reality this is not so, but during the day the diver's horizon is wider and he may not spot all that is there.
A trick worth remembering is that, because of the inability of low-angle sunlight to penetrate the water surface, the seabed becomes darkened (and nightlife becomes active) before it is fully dark at the surface. The clever night diver uses this fact to aid surface recovery by the cover party after his 'night' dive.
The use of a torch allows the diver to see easily under the water and, because the light source is close to the objects on which it is shone, the full spectrum of light is available to the objects on illuminate them. This means that instead of the usual blue-green look seen in daylight, the bright colours that actually exist are revealed to the diver. Plants and animals that look black by daylight are suddenly shown to be coloured in scarlets and purples. The same effect is, of course, available to photographers using artificial light and can lead to a rewarding pastime.
Night Diving Techniques
Night diving can provide a rewarding experienced but, to be done safely, it must be properly organized. All the standard rules of diving must apply and, in addition, there are several other things that must be in place before the dive marshal can be satisfied that everything will go according to plan.
Communication between divers can be maintained if both dives carry a torch and shine the torchlight on their hands as they use the normal diving signals. It is a good idea to draw the buddy's attention to the signal before it is made otherwise he may not notice it. Care should be taken not to shine your torch in your buddy's eyes since this spoils his night vision for some time. Buddy lines can be used on night dives, but usually the water clarity will be sufficiently good that they are not required. With a little care and practice, the diver should be able to keep visual track of his buddy by watching for his torch beam. If either torch fails during the course of the dive, it is a wise precaution to terminate the dive and to return to the surface, unless a spare is carried (even then it should really be used as means of ending the dive rather than continuing)
Communication from the divers to the cover party, whether they are in a boat or on the shore, is more difficult. Torches can again be used and the reommended lamp signals should be followed. Divers should avoid flashing their torches around on the surface, unless they are trying to signal, to avoid confusing the surface party. In sheltered and calm conditions (which are recommended for night diving) voice communication on the surface is fairly effective, although direction can be hard to judge.
The selection of a site for a night dive should be such that there are as many natural marks as possible which show the extent of the site and help prevent divers from becoming lost. A sheltered bay or cove provides an ideal place for a night dive, especially for those for whom it is a first-time experience. Other natural formations such as a cliff wall provide a very good guide on which to navigate out and back. The site should ideally not be too deep, since it is foolish to expose the divers to the rigours of deep diving on top of coping with darkness. Underwater incidents are much more difficult to deal with in the dark, hence it can be wise to limit the depth and keep it shallow, unless the divers are experienced night divers.
If divers wish to dive at night on a site with a more open aspect or with a moderate tidal stream flowing, then clearly they need to be covered by boat. There are two main techniques for the boat to keep in contact with the divers. Both involve the use of surface marker buoys. It is possible for the boat to tether to the diver's buoy and follow the divers around. The engine should not be used for this, as it would obviously endanger the divers when they surfaced. Consequently the cover party must be prepared to row, otherwise the boat pilot may inadvertantly drag the divers all over the dive site! The second technique is to mount a suitable light beacon on the surface marker buoy, which the cover boat then follows at a safe distance. In either case, the boat should not attempt to cover more than one pair of divers at a time.
It is important when night diving that dive marshalling procedures are at a premium and that everyone is accounted for, both when entering and leaving the water. The marshal should ensure that everyone knows and understands signalling and emergency procedures before the enter the water.