To install a marine stereo, mount it to either the dashboard area or in the cabin, make sure to protect it from water, and create support for the stereo well. Further, mount the speakers around the boat in locations that provide the best sound quality.
Marine stereos are typically mounted to either the dashboard area or to somewhere in the cabin. While they can be mounted vertically, they are designed to be mounted horizontally, as this provides the best protection for CDs and the unit when waves hit. When mounting the stereo, make sure to leave enough room in the back of the unit so that access is available, although once the mounting is done, the back of the unit should be covered.
Most marine stereos are somewhat waterproof, although water can still damage the unit. Plastic splash guards can be mounted over the stereo's faceplate to prevent water from splashing or spraying it.
The well, or area in which the unit is mounted, most likely needs to be supported, too. Stereo units can weigh a few pounds, and if they are held inside fiberglass, for example, they can easily break that glass if they bounce around enough. In general, if the supporting panel is less than .25 inches thick, it should be reinforced.
When mounting speakers, it's best to find locations that allow people to hear them from both ears. They should also be placed in locations where water can't directly damage them.