I was expecting a question about the use of the ocean for data center cooling since salt is corrosive to most metals. This is much better.
To expand a little, water acts as a solvent to dissolve minerals from basically anything it contacts. Basically any metal bonded with chloride ions is classified as a salt. This accumulates in the ocean. Fresh water basically comes from rain fall. This is a natural distillation process that removes the heavier solutes from the ocean water as it evaporates.
Over time this fresh water has removed the majority of easily soluble materials from where it commonly falls, thus fresh water aquifers and lakes, etc.
Fairly simplified, but what do you expect from a PC forum called Tom's Hardware.