A few source units will fully rotate without any signs of clipping with a 0dB signal (the highest possible gain from a CD). Most clip before that. 3/4 source unit volume is a bit conservative but still safe enough and gives you a ilttle latitude for program sources of varying gain. If that is what you set the system up at then you need to trust that level and not exceed it out on the water as the day grows longer and your auditory system gets tramatized.
While this isn't the most accurate way to set the system levels and is very simplistic, it's okay if you don't have a distortion detector or scope to read clipping.