why corrosion increases in saline water

The presence of salt (or any electrolyte) in the water accelerates the reaction because it increases the conductivity of water, effectively increasing the concentration of ions in the water and so increasing the rate of oxidation (corrosion) of the metal.

The situation is complicated by the fact that salt dissolved in water actually reduced the amount of dissolved oxygen present in the water. This is because the water molecules are attracted to the dissolved ions from the salt (solvation), which has the tendency to decrease the weak affinity of non-polar oxygen molecules to water, thereby driving dissolved oxygen out. So if the metal is totally submerged in water, higher salt concentrations can actually reduce the rate of corrosion, if the water is not aerated. So it depends upon the amount of oxygen getting into the water and onto the metal.

4Fe+3O₂+6H₂O⟶4Fe(OH)₃

Corrosion (rust) is 'redox' reaction, which means it involves reduction (of oxygen into hydroxide ions) and oxidation (of metallic iron to iron cations).