Installing a new marine receiver and speakers in your boat will give you an immediate upgrade in sound quality. But if you want to get the most out of your system, you need to add a quality marine-rated amplifier.
Most people associate amps with high volume. And while that's very true (and very necessary on ski boats and other open vessels), an amp brings more to your boat than just decibels – you'll also hear details. Instead of losing most of the notes to wind and engine noise, you'll hear your favorite songs the way you know they're supposed to sound.