Corrosion is the bane of materials engineers, boat and ship builders, and vessel owners alike. Prolonged contact to seawater and even the spray coming from the water's surface will corrode, degrade, and decay almost any substance, be it fiberglass, wood, or metal. Corrosive elements can get into vital mechanical parts and electrical wiring, and over time completely ruin the safety of a vessel. Constant vigilance is necessary to prevent marine corrosion from destroying a vessel in your care.
Marine corrosion has been around since the first proto-sailors first tried to float on hollowed out logs. Wood, even wood coated in pitch or some other substance designed to prevent the hull from becoming waterlogged degrades due to organic and chemical corrosion. It is an old trope to have pirates and seamen in movies talk about barnacles and the like in reference to their ships - barnacles and other sea life attach to ship and boat hulls and can slowly damage the hull.
With the advent of steel hulls and steam engines that can move ships across the seas at far faster speeds than in the days of sail and rigging, marine corrosion has taken on a new economic aspect. Ships are expensive and take months to years to build, and the wide variety of chemicals, organisms, and environments ships are expected to endure means that marine engineering has to deal with a wide assortment of corrosive substances.
Rust is a common sight on ships and boats that are not properly maintained, and if left untreated rust will eat away at the structural integrity of a vessel. The salt in the air and water in oceanic bodies contributes to the chemical degradation of exposed surfaces as well as the growth of corrosive organisms. Even the elements can assist corrosion by damaging ships, opening up previously sealed areas to the elements. Electronic components and wiring exposed to seawater can be rendered useless by marine corrosion.
The best defense against marine corrosion is proper care and maintenance. Cleaning vessels when they're at port or away from water is not enough - cleaning needs to be done even while underway if away from the dock for more than a few days. There is a reason that land based equipment must be modified before it can be successfully used out on the water - marine corrosion can damage sensitive components more quickly than you might suspect.