Salt water is unhealthy to drink not because of the size of the particles dissolved in it but because of the high concentration of dissolved salt. The oceans have an average salinity of about 3.5% or 35g/L., most of which is sodium chloride. Salt water causes dehydration When consumed. The salinity of the human body is only about 0.9%. Water will move out of cells via osmosis when they're in a hypertonic environment (higher concentration of solutes outside the cell than inside) causing the cells to shrink. This damages the cells.the kidneys use water to excrete the excess salt from the body and can use mire water than what was consumed.
Fresh water has a solute concentration closer to that of the human body. These solutes could be magnesium and calcium compounds in hard water, sodium chloride in softened water, chloramine used for disinfection and fluoride ion used to strengthen teeth. The solutes don't vary significantly is size from the solutes in ocean water.