Scuba diving is the sport of swimming underwater with specialized equipment that make it possible to breath underwater. The term "scuba", sometimes written as SCUBA, is an acronym for Self Contained Underwater Breathing Apparatus.
Even with proper equipment and clear conditions, humans cannot see as far in water as they can in air. Multiple factors impact overall visibility in water including refraction, desaturation, and turbidity. Visibility is typically expressed in feet or meters. Less visibility means increased danger from disorientation or from buddies becoming separated.
Objects underwater appear about 25% closer and 33% larger when viewed through a dive mask due to refraction. This can cause objects to be just out of reach when attempting to grasp them, despite appearing within reach.
Water desaturates sunlight, meaning colors are absorbed and become less visible as depth increases. Longer wavelength colors, starting with red, are the first to be absorbed at shallower depths followed by shorter wavelength colors as depth increases. At a depth of about 30 meters (roughly 98 feet) very few colors other than shades of blue and grey are visible to the human eye. Flashlights can therefore be useful to divers even during the day.
Turbidity is the quality of being cloudy or opaque due to particles suspended in the water, such as sand, mud, and algae. Turbidity can also influence desaturation. The greater the turbidity of the water, the more quickly colors are absorbed. The content of the suspended particles can also impact the visible color spectrum; for example, algae often shades objects in the water green. Shallow turbid water can therefore sometimes appear to have less color than deep, clear water.
Sounds typically heard while scuba diving include your own breathing, boats passing overhead, and clicks from fish that are feeding. Sound travels roughly 4 to 5 times faster underwater than it does through air due to waters density, which is roughly 800 times that of air. This makes the directionality of sound difficult to discern because sound waves arrive at both ears at virtually the same time. This can lead to danger, particularly because of the difficulty in determining the location and direction of boats operating nearby. Thus, the best thing to do while underwater and hearing a boat is to stay underwater at a safe depth away from the surface until the boat is no longer audible.
Body temperature underwater
Water conducts heat away from the body approximately 25 times faster than air, which is why a comfortable air temperature can feel uncomfortably cold in water at the same temperature. A wetsuit is therefore recommended for thermal protection in water temperatures under 30° celsius or 86° fahrenheit.
Thermoclines are abrupt changes in temperature between layers of water. They are formed because cold water is denser and tends to sink under warmer water when not mixed by currents and other forces. Thicker wetsuits are needed for colder temperatures, so knowing water temperatures at different depths can be an important factor in planning a scuba diving trip.
Tides and scuba diving
Tides are the periodic rise and fall in water level caused by the gravitational pull of the moon on large bodies of water. Depending on the tide, a dive site may be significantly shallower or deeper than it was on a previous visit. Tides also cause changes in currents that impact visibility and can make dive sites difficult or dangerous. The safest time to visit a dive site is generally during a slack tide which is the period between high and low tide and therefore usually the least amount of current. Tide tables, which detail the times for high and low tides and related water conditions throughout the day, are typically available in daily marine reports online from local news sources
Currents and scuba diving
Currents are the directional movements of water through a surrounding body of water in which there is less movement. A current can dangerously overpower a diver and carry them away, but can also be useful with appropriate planning. Drift diving is the practice of scuba diving while intentionally allowing the current to carry the diver from their entry point to a planned exit point some distance away.
Types of currents
There are several types of currents that divers encounter. Permanent offshore currents are currents that flow continuously in one direction, such as the North American Gulf Stream. However, divers are most often affected by longshore and rip currents.
Longshore currents are currents that run parallel to the shoreline and are generally caused by waves that approach the shore at an angle and influenced by tides and proximity to other currents. These influences can cause longshore currents to sometimes reverse direction, so awareness and careful planning are important when dealing with longshore currents. As indicated by the name, longshore currents can span long distances; thus when planning, the entry point of a dive in a long current should be well upstream of the planned exit point.
Rip currents are narrow flows of fast-moving water that usually run perpendicular to the shore. These currents are known for quickly pulling beach-goers, swimmers, and divers out to sea. Rip currents are formed by water flowing through the narrowing of passageways the water flows through, such as between sandbars or breaks in a coral reef. The best way to get out of a rip current is to swim parallel to the shore until free of the current.
Waves, surf, and scuba diving
Waves form as wind pushes on the surface of the water. As waves travel and encounter shallower water, the wave is lifted while the bottom of the wave slows down. As the wave steepens and the bottom slows, surf is formed when the wave topples over and "breaks" near the shoreline. Fetch is the measure of how long and hard the wind blows across the water unobstructed and is the main factor in the size of the wave and resulting intensity of the surf. Surf entry should not be attempted by scuba divers without special training.
Waves mostly affect divers near the surface, such as during shore dives or when getting on or off a boat, or in shallow water when they cause surge. Surge is the back-and-forth motion of water caused by waves overhead. If a surge is strong, divers are involuntarily moved by the motion of the water and have little control over the direction in which they move. To avoid the danger of being thrown against rock or coral formations during intense surge, it's best to move to deeper water where surge is less severe.