"However, just because you add 5k worth of speakers and a tower doesn't mean that your boat value will rise 5k."
And that's my point. He's not going to get his money back out of the upgrades. The boat itself might hold it's value or even go up a bit in value but if he puts $5K of upgrades in his $17K boat, it's still not going to sell for $22K later.
The best item to put on the boat for resale purposes is probably the tower. That usually gets you about $600-$800. The worst items are all stereo-related. My cousin is painfully learning that lesson now that he's selling his Sport Nautique. Nobody cares about his amps, subwoofers, or tower speakers. They make his boat attractive but don't make it worth more than a boat without those things.