For gel coat restoration, I have had great results using a heavy cut compound, a wool pad, and a rotary buffer set at 800 rpm. Find the third of the pad that gives best surface contact for the angle you're working and just keep the buffer moving.
For speakers, what is your battery and charging system setup? For one battery I think good option is two large speakers inside the boat with about 70 watts per channel amp. That is what I had in my 18' and it sounded great.
2006 Bayliner BR205
A1 Gen 2 1.47
5.0 carb engine