A marine stereo system has to be able to withstand a considerable amount of physical abuse. For this reason, specialized marine-rated equipment is used in saunas, spas, pool areas. Portable applications of a marine stereo system are on board boats and motorcycles, which also bring about potentially damaging conditions.
Salt is a common occurrence in the outside world that can cause substantial damage to stereo equipment. On any kind of craft that travels outside and does not provide a watertight chamber the way a car does, salt exposure is highly likely even if there is no salt water around. However, salt exposure becomes extremely prevalent near beaches in fresh water lakes and when marine stereo systems are on boats that are in the ocean.
Salt's primary issues are that salt can chemically tarnish electrical equipment and the physical stress of salt can erode away important connections. This can result in shorting out equipment and rendering it completely inoperable. For this reason, marine stereo equipment is specially treated with purpose-built coatings that are designed to resist salt. Further, once the coatings are applied, the testing procedures of marine stereo components involve having them sit in a salt fog that simulates extended exposure to highly salty conditions like those of the open ocean.
Most kinds of electrical equipment will short out immediately if water bridges the area between an electrical load and its power source. This will cause the device to no longer function correctly, and in many cases it must be replaced. Further, the water can also carry minerals that will have the same impact as salt will. The bare minimum standard that must be met is an external case that renders a piece of equipment "water resistant." This means that splashes and light rain are okay, but it does not protect against submersion. When a piece of electrical equipment can be submerged without being damaged, this is referred to as being "waterproof." Marine stereo equipment has to be waterproof by definition, as even boats tied to the marina are under constant assault from storms and splashing conditions.
Any external case or remote control for a piece of marine stereo equipment must be highly resistant to ultraviolet radiation. Otherwise, the best case scenario is fading of the finish. In many cases, without UV resistance the casing will crack and fall apart. Special coatings protect against UV radiation.