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Seawater corrosion is a form of corrosion of metal exposed to seawater. Typically in such cases the metal is a structural component of a vessel (ship or boat) or a fixed structure either on the shore, offshore, or underwater. In these cases, seawater corrosion typically acts on a time scale of months to years. Corrosion is faster with higher salinity and to a lesser extent higher temperatures.

Vessels that are only in seawater temporarily, e.g. trailerable boats, often do not require much corrosion protection. Structures that do need to be in seawater over the longer term are often protected with one or more pieces of zinc. Since this zinc corrodes faster than almost any other metal (see the Galvanic series), these pieces of zinc are referred to as sacrificial zincs and must be replaced periodically, with the replacement interval possibly being as short as a few months. In general, sacrificial zincs should be inspected frequently, at least until a baseline rate of loss is established, and then at regular intervals based on this projected rate of loss. Often, different components of a vessel or structure are each protected by their own zinc. For example, the cooling system of an engine, the propeller shaft, any metal rudders and, for metal-hulled boats, the hull itself are each protected by one or more zincs.

It has been suggested that vessels in a harbor or marina with electrical connections to shorepower are all at approximately the same ground potential, so the zinc on one vessel may help protect other vessels (which may lack such protection), and may corrode correspondingly faster.