Nowhere in nature can absolutely clear water be found. Even rainwater contains dissolved minerals. For example, 30kg of rainwater contains about 1g of solid substances, whereas 1kg of seawater contains about 35g different salts. Salinity is a very important characteristic and it directly influences the viability of the organisms living in the water. The total amount of dissolved salts in the oceans is 48 quadrillion tonnes. Most of it is NaCl (77.8%), to which the saline taste of seawater is due. Magnesium chloride accounts for 10.9% and it gives the water a specific bitterish taste. Then come sulfates (10.8%), carbonates (.5%), etc.  The overall amount of salts may vary in different regions of the Ocean but the composition and the percentile content remain the same. The average ocean salinity is 35% and increases starting at the Equator and reaching 15–20º latitude, then it decreases towards the poles. This phenomenon can be explained with the distribution of water vapors and also with the saline waters transported from the tropical zones by warm currents. Salinity in the northern part of the Indian Ocean changes according to the season. Seas have greater salinity than oceans. The Dead Sea has the greatest salinity of all (256%), and the smallest is that of the Baltic Sea (5–7%).

The presence of more than 44 chemical elements has been established so far. Besides the popular oxygen, hydrogen, chlorine, potassium, magnesium, sulfur, calcium, and iodine, seawater also contains Al, Cu, Zn, Tn, Pb, Au, Ag, U, Mn, Hg, etc.

There are many gases dissolved in seawater, the largest amount being contributed by oxygen, nitrogen, and carbon dioxide. Water dissolves oxygen better than nitrogen. That is why the ratio N:O which is 4:1 on dry land, changes to 2:1 under water.

Relative Weight
It is interesting to note that 1liter of air weighs .013N and 1liter of water weighs 10N, that is 770 times heavier than air. The relative weight of seawater depends on the density and temperature of the water. Density itself, though insignificantly, depends on temperature. That is why at 20ºC the density of the water is lower by .2% than it is at 4ºC. Pure distilled water has a relative weight of 1 at a temperature of 4ºC, that is, 1 cm³ of water weighs 1g. Seawater is heavier than fresh water by 2.5–3% because of the greater amount of salts dissolved in it; its relative weight is 1.025. It may be concluded that a diver weighs less in seawater than in fresh water. Relative weight is important for determining buoyancy.

Resistance
Just like any other liquid, water practically does not shrink. That is why its density almost does not change at different depths. At a pressure of 500at water shrinks by 1/47,000,000 of its volume. If it did not shrink at all, however,  the sea level would rise by 30m. Water resistance is greatest in the surface layer. Therefore, less effort is needed for swimming in that layer.

Transparency
The relative transparency of seawater is determined by the average depth, at which a white disc of a 30cm diameter is no longer seen. Greatest transparency has the Sargatian Sea (66.5m), second greatest transparency have the Syrian coasts of the Mediterranean Sea. Least transparent is the North Sea (the British Channel) – some 6.5–12m.

Temperature, Heat Capacity and Heat Conductivity
The heat capacity of seawater is 3134 times greater than that of air. Water has insignificant heat conductivity. That is why distribution of heat to greater depths is very slow and is mainly achieved through convection.

The highest temperature of water is registered to occur between 3 and 4 p.m., and the lowest – a couple of hours after sunrise. There are three temperature layers of seawater: surface layer (epilymnion), intermediate layer (metalymnion), and deepest layer (hypolymnion). The thickness of the former two layers varies with the weather, season, and currents. The temperature of the surface layer is almost constant, being between 19 and 25ºC in the summer. As the deepest layer begins, temperature drops by a few more degrees and it remains constant thereafter (7–9ºC). That is the temperature of sea depths and it does not depend on the season.

Water Motion
Water motion constitutes sea currents and waves. The reason for the formation of currents might be the different density of water, constant winds, etc. Ocean currents are usually caused by constant winds, whereas local ones are mainly due to the character of coastlines. According to he direction of their flow, currents can be classified as vertical or horizontal. There are three main types of waves: wind waves, standing waves, and seismic waves.

Wind is the main reason for the formation of waves. The process of wave formation can be divide into different stages. When the speed of wind is less than 1m/s, air motion does not affect the surface of the water. If wind intensifies, these rows of waves become irregular and peaks appear, which are due to the different pressure at the front and at the back of the wave. At a greater speed of wind large waves are formed, running in parallel rows. Th largest waves reaching hundreds of meters continue even when the wind has ceased. They create the so-called dead drift. 

Standing waves are formed when the level of the water rises at one coast and in the same time drops at the other. A sudden decrease of atmospheric pressure at one of the coasts, appearance of strong wind or heavy rain can all be the causes for standing waves. The fluctuation of the sea level may reach 80cm, which is dangerous for vessels at the harbors.

Seismic waves are formed because of underwater earthquakes. A vessel that is nest the site of the earthquake experiences a hydraulic blow which is why old maps frequently contain non-existent reefs. Seismic waves are often present in the Hawaii region where they have the special name zunami. Such waves are formed in the Pacific Ocean, the Mediterranean, the Caribbean Sea, and the Malaya Archipelago as well. Sometimes, these waves reach the height of 35m and are dangerous not only for the ships but also for the native population because of their destructive power.

Difficulties
The change of water density, resulting from changes of temperature, salinity, and pressure has no practical importance to diving. Even though, water is a dense medium and creates significant difficulties for a diver’s movements. He or she cannot walk or turn as fast as in the air. While working under water, divers must choose positions and movements that create least resistance, e.g. walking sidewards being slightly bent forward.  The use of tools is also hindered. For example, the use of a hammer is much more difficult under water than it is in the air. As a result, divers quickly get tired. That is why the work that is to be performed under water should be organized so as to facilitate the diver by minimizing unnecessary movements and providing possible help from the surface. Rapid currents additionally impede the accomplishment of underwater work. Mire, too, can create considerable difficulties for divers. Even the execution of simplest types of work becomes complicated and requires dexterity, resistance, and fitness. That is why rigorous physical preparation is crucial.