When steel is immersed in water with a small amount of salt, significantly accelerated corrosion will occur. Natural sea water salt concentration is said to be an average of 3% (30g/L), though there are variations based on locations, and is the most corrosive of all natural environments. The corrosiveness increases as the water flow rate increases. The actual corrosions vary depending on locations/seasonally since the natural sea water is quite complex in composition containing other corrosive components other than salts such as dissolved oxygen.

When examining the corrosion of steel by sea water, the rate of corrosion is far more severe when steel is repeatedly immersed and pulled out, compared to just being left immersed in water. For instance, consider steel pilings used as shore protection for ports.
Steel pilings are driven into the ocean floor along the shore-line, and contact the ocean floor mud extending through the sea water to the water line. The most severe corrosion occurs at just above the water line where the waves splash. This area is called the "Splash Zone". In the water, dissolved oxygen amount is limited and corrosion progress would be limited, but abundance of oxygen is supplied from the atmosphere in the Splash Zone and the corrosion progresses rapidly. The corrosion progresses more rapidly out of the water, compared to being immersed under the water, since the salt deposits on the metal surface attracts humidity coupled with abundance of oxygen supply through thin layer of water remaining on the metal surface.
The fact that corrosions decelerate after the metal is pulled out of fresh water but accelerates with sea water, means the corrosions are promoted when metals are wetted with sea water in the atmosphere.

For other marine structures not limited to the steel pilings such as offshore airports, connecting bridges, marine city structures, and offshore oil rigs are subject to Corrosion protection countermeasures. The countermeasures are classified based on applicable corrosive environmental factors such as air, plash zones, tidal zone, sea water, ocean floor soil, etc.
For the most severe corrosion environment of splash zones, wrapping of corrosion resistant alloys such as 2~3mm thick Monel (70% Ni, 30% Cu) or pilings made of Monel clad metal are used, since normal paint with 200~300 Micrometer thickness film would be ineffective.
Recently, organic linings as thick as 10mm of resin mortar, tar epoxy, and urethane rubber are used effectively.
For corrosion prevention in electrolyte (sodium chloride, etc.) rich sea water and marine soil environments, electrical corrosion protection measures are in use. (Electrical corrosion protection measures are discussed later)