unlike what happens in continental waters, which can have a very variable composition (hard, soft, brackish, etc) sea water is characterized by a relative homogeneity in terms of the substantive salts that it has dissolved, although it can vary the proportion in which they appear, that is, their salinity.
that uniformity, however, occurs only in short geological intervals, since from its origin to the present day the composition of the water of the oceans has undergone various variations with the corporation of new elements and their distribution throughout the planet. the currents and the slow diffusion processes in this more or less continuous medium, even when the points of communication are reduced, are responsible for the phenomenon.
in closed seas, such as the Baltic or the Mediterranean, with a very narrow exchange channel with the corresponding ocean, conditions of uniformity take longer to reach and certain differentiating features always occur.
The main elements present in marine water are, in order of quantitative importance, chlorine, sodium, magnesium and calcium, followed by practically all of the rest that exist on the planet, even if some of them are almost vestigial. all these elements are presented in the form of salts, the main ones being, also in order of quantitative importance, sodium chloride, magnesium chloride, magnesium sulfate, calcium chloride, potassium chloride, sodium bicarbonate and sodium bromide.
Water is usually spoken of when the amount of these salts is at least 17%, it is brackish water, which is found in coastal waters, marshes and river mouths.