Marine animals live in a salty (hypertonic) environment, and they tend to lose water to the environment by osmosis. So, ocean animals have adapted to salty water in a few ways - e.g. getting rid of solutes by active transport through gills, water loss by osmosis through the gills, and minute amount of urine containing solutes eliminated by excretory organs.
In most of the oceanic areas the salinity of the sea water is 3.5 parts per hundred. This measurement is a total of all salts that are dissolved in water. All the salts are dissolved in even proportion with respect to other. The major proportion was observed to be the table salt which is nothing but the sodium chloride, mixed with other salts.
There are several factors that are responsible for the variations in ocean salinity. The most common factor is the relative amount of evaporation or precipitation in an area. If suppose the evaporation of water from the sea is more than than the precipitation then the salinity increases. If there is more precipitation (in the form of rain), than evaporation then salinity decreases.
The freezing of ice while forming icebergs also effects the salinity of the sea. As the icebergs form they increase the salinity of seawater where as some of the icebergs melt which give rise to decrease in the salinity.
Many marine organisms are highly affected by changes in salinity. This is because of a process called osmosis which is the ability of water to move in and out of living cells, in response to a concentration of a dissolved material, until an equilibrium is reached. In general the dissolved material does not easily cross the cell membrane so the water flows by osmosis to form an equilibrium. Marine organisms respond to this as either being osmotic conformers (also called poikilosmotic) or osmotic regulators (or homeosmotic).