Sea water

The composition of sea water and the body fluids of many animals are alike. This makes this saline environment the most appropriate for living cells, containing all the elements essential for the growth and maintenance of protoplasm.

Which brings us to the question of the composition of sea water!
In the open ocean, sea water has an average salinity (saltiness) of around 35 (34.7) o/oo (parts per thousand), which is the same as 3.5 %. In oceanography, the salinities are measured in o/oo, or simply by moving the decimal point one space to the right to change % to o/oo. The salinity is simply the content of the dissolved salts in water, or the measured amount of solids dissolved (in grams) in a given amount of water (one kilogram)--this gram per 1000 grams makes salinity be given in parts per thousand.
Since the proportions of the major chemical constituents of sea water is remarkably uniform in most parts of the open ocean, measuring only one of the major components can determine the salinity by calculation. Chemical/physical oceanographers used to measure salinity by determining the concentration of chloride by chemical means. However, today, salinities are more rapidly and easily measured by determining the electrical conductivity of sea water with a device called a salinometer.

The major constitutents of sea water are sodium and chloride (hence, salt water is usually given chemically as NaCl, which are the symbols for sodium/Na and chlorine/Cl). However, sea water, as given earlier as water being the "universal solvent," contains many, many other elements. Other significant components include sulfates (SO4), Magnesium (Mg), Calcium (Ca), Potassium (K), and bicarbonates (HCO3). The ratios of these salts to one another do not vary much in the oceans or over time periods. Those salts and elements that do not vary much over time in an area are called "conservative elements"--these are sodium, magnesium, calcium, potassium, chlorine, sulfate, carbonate, bicarbonate, bromine, boric acid, lithium, rubidium, barium, aluminum, uranium, lead--in fact, more than 99.9% of all the dissolved salts.
On the other hand, some minor constituents show marked variation in relative concentrations over time, due to their selective removal from the water by living organisms. These are called "non-conservative" elements--phosphate, nitrate, and silicon. These 3 elements/compounds tend to be in short supply in surface waters where they are taken up by plants, but their concentrations increase sharply below the depths of photosynthesis as a result of their release into the water from the decomposition of sinking organic particles.
The ratio of major salts to each other, and usually in total concentrations as well, are similar in sea water and in the body fluids, especially of the marine invertebrates (those animals without a vertebral column. But this similarity is not restricted to just the marine invertebrates but extend out, more or less, to include "higher" (and presumably more complex) animals/vertebrates, such as fish, amphibians, and mammals.
There are some variations noted to this general similarity, but they are local in sites, affected by climate or the weather. One particular location in the oceans where one can expect and do observe considerable variation is the edge of the oceans, which most of us explore as the tide pools.
To date, the major areas of concern with the chemical content of the oceans have been in the shallow seas (less than 200 meters of depth), but since these are the areas of heaviest biological productivity or human contact/significance, the concerns voiced have not been insignificant. In the Pacific Ocean, many of the coral reefs in the central and southwestern Pacific are relatively "clean," but the northwestern line from the Philippines to Japan are critical due to overexploitation and pollution. Some of the coral reefs associated with the South Pacific islands that compose Melanesia, Micronesia, and Polynesia, are, at best, threatened and bear corrective measures.