When seawater or water in the ocean has reached the point where it cannot hold any more dissolved minerals, these minerals precipitate out of the water.
This means that they separate from the water in a solid form again.
The minerals then sink to the ocean floor. Saturation usually results from evaporation of the water. As the amount of pure water is reduced, the amount of minerals that remains is more than the water can hold.
Different minerals separate from water at different points. Calcite and dolomite precipitate out first, followed by gypsum. Halite, sodium chloride in rock crystal formation, then separates from the water.
Magnesium and potassium are the last minerals to precipitate out of water, if the process gets that far.
If all the water evaporates, the minerals are left in evaporite deposits. The Mediterranean Sea is a good example of salt deposits underwater. Because the sea is almost completely landlocked, more water evaporates than enters.
The seawater, therefore, has a high salinity rate and thick salt deposits on its floor.