Why are marine organisms affected by salinity?

Answer 1

Salinity is relative. Fresh water has salinity, just not very much. Oceans have higher concentrations of salt than fresh water. Oceans have different salinities in different areas - where water evaporates quickly, where rivers empty into the ocean, where icebergs are melting, ...

Organisms have to maintain homeostasis as far as water and salt are concerned. Individual organisms are adapted to certain conditions, and higher or lower salinity levels will affect them adversely.