Okay I have tried different RCAs and even switched front and rear RCAs and the noise continues through all speakers. This points to the noise coming from the head unit. So my question is how can I determine whether the head unit is just not grounded well (visual inspection looks fine) or there is something in the head unit causing the noise and the head unit needs to be replaced? This boat is a 2011 so it is not like the head unit is old.