A simple potentiometer could be used to control the line-level signal going to an amplifier (and then to speakers). I did this once in my car to control the subwoofer volume. That was mono and you need stereo, of course, so you will need a stereo pot, or you could just use two. I bought mine at Radio Shack for something like $3. A stereo one would be a little more expensive but a better way to go. Go to
formatting link
as they will have what you need. You basically just need a stereo line-level attenuator between your HU and the amp that will drive these speakers. Just split the signal that is going to that other amp to get the line-level signal for the stereo attenuator and then you will have a volume control for this new amp/speaker combination you want to add to your boat.
Hope that helps,
MOSFET