As a full-time musician/singer/entertainer for more than a half century I owned more sound systems than anyone can possibly imagine. The list is endless, the systems ranged from huge to immense, speakers weighing in excess of 100 pounds, amps to nearly 1,000-watts, the works.
Not too many years ago Bose came out with their L1 series of professional sound systems. The L1 was originally designed for guitar players who also sang. The system was then modified for arranger keyboards, synths, etc..., and was later coupled with an incredible 4-channel mixer.
It took a lot for me to dump my five-year-old Barbetta Sona-32CS sound system, then shell out nearly $2,500 for the Bose L1. After then first performance I never looked back. Bose IS THE BEST--no doubt about it! Bose sound systems are considered the Gold Standard that other manufacturers try to emulate, and despite the high price-tag, they're worth every penny.
Someone suggested flush-mounting the speakers outside, then building a box around the back side of the speaker so water couldn't leak into the boat. The only problem with that is speakers, all speakers, are mounted inside acoustically balanced housings that are vented for maximum movement of the speaker cone. The quality of the sound is highly dependent upon the speaker housing, even those used for automotive purposes. For example, speakers mounted on the back deck of a car utilize the trunk's interior shape, size and components to drive the sound out of the speaker cone itself. Same holds true with speakers mounted in car doors and side panels. The cavity behind the speaker is equally as important as the speaker itself for producing the ultimate sound quality.
Of course, there are some folks that just want it LOUD. For those individuals the industry produces massive sub-woofers, and long-throw speakers that move huge quantities of air. The speakers are connected to massive, solid-state amps that drain the boat's battery system within a few hours. What is often emitted from the speakers is horrendous, thumping sounds at ear-bleed volumes that can be heard for miles. You'll find examples of this every summer weekend at Fairlee Creek and similar locations throughout the mid-Atlantic region.
One more word about Bose. From my perspective, they have the best customer service of any manufacturer I've dealt with in decades. I know of several musicians that had audience members damage their systems and Bose had another system to them the following day. If there was a warranty problem, they usually replaced the entire system at no charge whatsoever. Now that's service with a capital "S."