There are many types of boats, in different sizes and styles. In addition we all have our different needs and requirements. BUT, the foundation (blue print) is still the same. For a good marine install the design of the system is as critical as the components and/or installation.
When designing a balanced sound system In a marine environment, we need to look at a few requirements such as the source (head unit/radio), speakers, subwoofers, amplifiers, installation and more carefully.
The Source: There are many manufacturers that are offering marine audio units. Our choices vary from a simple AM/FM/CD units to Am/Fm/CD controller/MP3/XM digital satellite radio and more. CD changers tend to skip less than in dash units. It is important to replace the radio with a unit that will have proper signal/preamp to control external amplifiers, preferably 4 volts or greater (preout). The high level preout will also run the audio system more efficiently. Having three pairs of pre-amp RCA output will allow us to add a 5 or 6 channel amplifier with separate controls from the radio to each speaker, especially the subwoofers. Good LCD or other displays are very important in overcoming the direct sunlight.
Amplifier: The amplifier is at the heart of the audio system, giving us the full and clean power to run the speakers properly. A well-balanced setup will consist of at least one 4 channel amplifier. One half of the amplifier will run the 2 or 4 speakers (6"x9" or two ways) using the internal cross over as high pass (100 hz), and the other part will power the subs using the low pass (80 hz) cross over. The speakers will play in stereo while the subs will be bridged mono. It is important to utilize an amplifier that is stable at 2 ohm and wire the speakers for 2-ohm load. The charging systems in boats are low and power is limited; therefore, efficiency is very important.
Speakers: Every boat will have at least four 2-way speakers. The speakers should be mounted as high as possible with some space behind. XTC baffles can be use to isolate the rear of the speakers from the open area. In some cases, using components will assist in lifting the staging upward. Marine speaker baskets or frames are usually coated (white) to prevent rust, and the speakers are made from polypropylene. They are all considered water resistant and will last for a few seasons.
Subwoofer: Adding at least two 10" subwoofers to the audio system will bring the sound to complete. It will enhance the lower frequencies. Since bass is non-directional, it can be installed in many positions/locations of the boat. A few manufacturers are building special subs for the marine environment. With some subs, the area should sealed (build enclosure) to house the subs. Some are designed to work in an area under seat compartments without designing a special enclosure. That's known as infinite baffle.
Battery: Adding a separate battery to run the audio system is important. Using a deep cycle battery with proper protection as well as a master switch or 200 amp solenoid isolator will allow the system to play off the auxiliary battery without interfering with the main battery. Therefore, the latter will always be available to start the boat regardless of the auxiliary battery power availability.
Please note that when adding multiple additional batteries the alternator must be upgraded as well as the charge line.
Antenna: There are hidden antennas that can be used to assist in receiving AM/FM. In addition, XM Radio antennas for marine are becoming available. Grounding the antenna lead to a rail works very well.
Options: I highly recommend to use XM Radio for boats. You will find all you need without having to deal with skipping CD's. With XM Radio there are 100 digital radio signals with no interference anywhere on the water and with limited commercials on most of the stations.