Here's Plan B... 4 6 1/2" speakers run straight off the radio and a pre-powered sub in the cuddy... Will that be loud enough to still comfortably hear it while running?

very much so
above decks on my 30'er I have two 6.5's running in series with two more in the v berth sharing 30 watts rms between all 4 and 2 6x9's at 15 watts rms each in the back of the cabin.... I am I/O with 2 mercruiser 260's and even WOT at 48mph I don't need more than half volume to hear it clearly.... I hate having to shout over the radio to talk to others while driving.