Seawater is more than table salt in water

What the saltiness actually measures

The familiar taste encourages a simple picture: ordinary salt poured into an enormous bowl. The USGS describes a more complicated mixture. Rain and runoff weather rocks, transporting dissolved ions through rivers to the ocean. Some ions are taken up by organisms; others remain in the water much longer. Sodium and chloride are the most abundant ions, but they are not the whole mixture.

A commonly used approximate ocean salinity is 35 parts per thousand. On a mass basis, that is about 35 grams of dissolved salts in a kilogram of seawater. It is not 35 grams of salt added to a kilogram of fresh water, because that would change the denominator.

Sources: s1

The seafloor participates too

NOAA identifies openings in the seafloor as another source. Water circulating through hot rock exchanges dissolved substances before returning through hydrothermal vents. Underwater eruptions also release minerals. This is a chemical system with several pathways, rather than a single deposit of salt that simply dissolved all at once.

Sources: s2

Keep the units attached

Copy the reported quantity, units, sampling location and date before attempting to compare two reports. A total-salt concentration, a chloride concentration and an electrical-conductivity reading are different observations. Treating them as interchangeable could manufacture a dramatic change where the reports simply measured different things. Anyone reconstructing an island's water supply, or inventing one for a story, needs to state which quantity the account uses.