A sea is a large body of salt water. It may be  an ocean or may be a large saline lake which, like the Caspian Sea, lacks a natural outlet.

Characteristics

Seawater 

Seawater is salty. The open ocean has about  solids per litre, a salinity of 35ppt. The Mediterranean Sea is a little higher at 37ppt and the Dead Sea has as much as  dissolved solids per litre (300ppt). Although sodium chloride is the main salt present, making up about 85% of the solids in solution, there are also  per litre of the chlorides of other metals such as potassium and magnesium and  of sulphates, carbonates, bromides and other salts. A kilogram (2.2 lb) of salt can therefore be found in 28 litres or one cubic foot of typical ocean water. Despite differences in the levels of salinity in different seas, the relative composition of the dissolved salts is very stable throughout the world's oceans.

Temperature 
The temperature of the sea depends on the amount of sunlight falling on the surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over  while near the poles the temperature in balance with the sea ice is about . Cold water is denser than warm water and tends to sink. Cold water is denser than warm water and tends to sink. There is a continuous circulation of water in the oceans. Warm surface currents cool as they move away from the tropics, the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and  density of the water, eventually welling up again towards the surface. Deep sea water has a temperature between  and  in all parts of the globe.

Oxygen 
The amount of oxygen found in seawater depends mostly on the plants growing in it. These are mainly algae, including phytoplankton, but also include some vascular plants such as seagrasses. In daylight the photosynthetic activity of these plants produces oxygen which dissolves in the seawater where it is used by marine animals. At night, photosynthesis stops, and the amount of dissolved oxygen declines. In the deep sea where not enough light penetrates for plants to grow, there is very little dissolved oxygen.

Seawater is a little alkaline and during historic times has had a pH of about 8.2. The pH is expected to reach 7.7 by the year 2100, an increase of 320% in acidity in a century. One important element for the formation of skeletal material in marine animals is calcium but it is easily precipitated out in the form of calcium carbonate as the sea becomes more acid. This is likely to have profound effects on certain planktonic marine organisms because their ability to form shells will be reduced. These include single-celled algae called coccolithophorids and foraminifera. These are important parts of the food chain. Reducing their numbers will have significant results. In tropical areas, corals will be affected by a lack of calcium, with knock-on effects for other reef residents.

Waves 

Wind blowing over the surface of a body of water forms waves. The friction between air and water caused by a gentle breeze on a pond causes ripples to form. A strong blow over the ocean causes larger waves as the moving air pushes against the raised ridges of water. The waves reach their greatest height when the rate at which they travel nearly matches the speed of the wind. The waves form at right angles to the direction from which the wind blows. In open water, if the wind continues to blow, as happens in the Roaring Forties in the southern hemisphere, long, organized masses of water called swell roll across the ocean.

References

Other websites 

 
 National Oceanic and Atmospheric Administration   
Ocean and sea

 
Bodies of water