Transcript: Do you need a subwoofer? That is actually a fairly subjective question that depends a lot on your listening habits and your personal taste.
It is very possible to get excellent bass response from quality full range marine speakers without a subwoofer. In fact, some would argue that too much bass is not good or accurate sound reproduction.
Others want to feel the music as much as they want to listen to it and they are looking for a thump that shakes the harbor. That is exactly what a subwoofer can give you that no full range speaker can hope to imitate.
If you plan on using a subwoofer, then you must plan on powering it adequately. An underpowered subwoofer is pretty much the same as no subwoofer at all. You should plan on dedicating anywhere from 100 watts to 500 watts or more to your subwoofer and that usually means an external amp, although some subwoofers have a built in amp of their own.
Bass frequencies are very persistent so subwoofers are not very directional. Generally you want to mount them as low as possible and the direction they point is not very important.
A subwoofer should always be used with a crossover or frequency filter. A crossover directs only the lower sound frequencies to your subwoofer. Running full range sound through a sub can make your music mix sound like mud. Fortunately, most amplifiers have crossovers built in, but you will need to follow the instructions on how to set it up.