You could certainly buy some better speakers but if you want to put them in the original locations they will have to be the same size which limit's the "improvement" you would hear, assuming the original speakers are in good condition. By now it is likely that the foam surrounds on some or all of them has rotted away to nothing. The two speakers in the guest room had done that though the rest were OK. I just put in new surrounds for a few bucks. It's not difficult.
I am a serious audio nut at home, with a fair bit of "classic" equipment but, for some reason, have no particular interest in having "great" sound on the boat. I thought about putting one of my speaker systems on the boat but I didn't want to take up the floor space and, as I said, I just couldn't see any point in it.
So the boat is oem except for the fact that all the music is supplied by an Ipod.