The ability to audibly signal the presence of your craft on the water is a legal requirement. The most common way this is done is with a horn.
Most boat horns run off of 12 VDC. The sounding mechanisms they employ are an electrical diaphragm (like a car horn), a piezo (like an emergency buzzer) or an air diaphragm with a compressor (like a truck or ship horn).
A horn is a crucial safety feature when a sudden warning needs to be given, to signal trouble, or when underway with low visibility such as in fog.
Boat horns are available from several well known suppliers, the most famous in marine being the AFI division of Marinco. As the say on their website, they are
…the leading supplier of horns to the marine industry, with over fifty years of experience in designing and manufacturing sound devices specifically for use in harsh marine environments. Stainless steel is used for all critical components such as trumpets, motor cover, diaphragms, and assembly and mounting hardware. In the case of the XLP trumpet horns, the horn is given extra protection through the complete over-molding of the internal motor cover housing. It is this extra process that provides the added protection needed to back up the five-year warranty. AFI offers a complete line of marine horn products designed to meet almost any need, including electric and air trumpet horns, compact horns, electric and air below deck horns, and a comprehensive line of drop-in horns with a wide assortment of grill options.