there are many dissolved ions in sea water that make up the salts in water; sodium, chlorine, potassium, calcium, etc. On average the ocean has a salinity of 35 ppt(parts per thousand) or 3.5% The salts mostly originate from the weathering and erosion of rocks from land and in the ocean itself. All of the major ions exist in a constant proportion so that no matter what the exact salinity, there will be the same ratio of ions. salt water is usually higher where there is high evaporation or at the poles where fresh water is removed but the salts remain in a higher concentration. It is usually lower near river mouths or areas of high precipitation.