Encyclopedia Britannica details on this query:
"The density of a material is given in units of mass per unit volume and expressed in kilograms per cubic metre in the SI system of units. In oceanography the density of seawater has been expressed historically in grams per cubic centimetre. The density of seawater is a function of temperature, salinity, and pressure. Because oceanographers require density measurements to be accurate to the fifth decimal place, manipulation of the data requires writing many numbers to record each measurement. Also, the pressure effect can be neglected in many instances by using potential temperature. These two factors led oceanographers to adopt a density unit called sigma-t (σt). This value is obtained by subtracting 1.0 from the density and multiplying the remainder by 1,000. The σt has no units and is an abbreviated density of seawater controlled by salinity and temperature only. The σt of seawater increases with increasing salinity and decreasing temperature.
The relationship between pressure and density is demonstrated by observing the effect of pressure on the density of seawater at 35 psu and 0 °C. Because a one-metre (three-foot) column of seawater produces a pressure of about one decibar (0.1 atmosphere), the pressure in decibars is approximately equal to the depth in metres. (One decibar is one-tenth of a bar, which in turn is equal to 105 newtons per square metre.)
Values associated with the change in seawater density with depth are listed in the table." [See link for table]
http://www.britannica.com/EBchecked/topic/531121/seawater/301668/Density-of-seawater-and-pressure
Dear @Sundar, the density of sea water depends on temperature, salinity, and pressure. "The mathematical relationship between these factors in somewhat complicated, and is known as the “equation of state” of seawater. There are several online calculators,though, that compute density of seawater from input values of temperature, salinity, and pressure.
The density of seawater is always greater than 1.000 g/cm3 and less than 2.000 g/cm3,
so oceanographers often express density as sigma, which is [(Sea water density - 1) x 1000]..."
http://oceanexplorer.noaa.gov/explorations/04mountains/background/edu/media/MTS04.density.pdf
Fresh water influx at the continental margin can also influence on the density. The availability of nutrients as well. Climate, ocean depth, seasons, currents...
Dear @Sundar, the density of sea water depends on temperature, salinity, and pressure. "The mathematical relationship between these factors in somewhat complicated, and is known as the “equation of state” of seawater. There are several online calculators,though, that compute density of seawater from input values of temperature, salinity, and pressure.
The density of seawater is always greater than 1.000 g/cm3 and less than 2.000 g/cm3,
so oceanographers often express density as sigma, which is [(Sea water density - 1) x 1000]..."
http://oceanexplorer.noaa.gov/explorations/04mountains/background/edu/media/MTS04.density.pdf
A short tip, don´t forget the uptaken gases like CO2, CH4 etc. And of course you just could mention the depth from surface and streaming pressure.
While it would be correct that the density of sea water depends on temperature, salinity and pressure. The correctness of this proposition is dependent on the appropriateness of using a reductionistic approach which can also be viewed as over-simplistic and misleading. There is some irony in those situations when people use very high level of precision instruments to calculate this density while at the same time simplistically using the formula mentioned. As some people already hinted towards, for example the chemical content of sea water (including presence of different gases and their relationships) will also influence. Furthermore bio-chemical content and micro-particles will contribute (and it is not all about salinity). In other words - content of *water* - or the answer to the question - what is the actual content of this particular sea water? together with the question - where is this particular sea water? needs to be taken into consideration - and it is not necessarily always advice-able to make universal and generic assumptions. All seawater is only equal with a simplistic definition if you exclude the contextually dependent exceptions. The real question becomes - what do you intend to use the the answer to your question for? It is not irrelevant...
I would also argue that the density of seawater is not *always* more than 1000 g/cm3 - which is why a few exceptional examples of ships *mysteriously* have sunk without any *normal* reason (e.g. they just could not stay afloat). Just because the density of seawater is normally more than 1000 g/cm3 does not make it necessarily always so. *Abnormalities* do happen, temporal and very unusual perhaps - but not impossible.
We have to take in mind the point where we want to find the density of the water. Many factors affect the density: pressure at the depth of the point of measurement, temperature, aeration and air contents in bubbles, any river stream, pollution, rate of evaporation of the surface water, water streams, percentage of living creatures there, and many more. It is a very difficult task if we want to model this process.
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