First eliminate the head unit for noise by unplugging your rca's at your amps.
If noise is gone, reconnect and unplug again but this time at the head unit. You'll now eliminate the fact the noise could be picked by the interference through the rca's. If sound is gone again, with rca's unplugged from head unit, take a small.piece of wire and touch the head unit chassis to the rca ground at the head unit. If you hear a change or the noise is gone, you have a ground loop and adding that little wire can be used if noise is gone. If noise goes away a little bit using the little wire, that's not the main issue. Now, go to the amp. Use same little ground wire and touch to rca ground and amps grounding point where amps ground wire connects to amp. Not the ground point where the wire picks up power from.
If noise goes away, there's your problem, a ground loop.. if it does not go away,, ground rca's to the main ground point where amps gets its main connection from, where ever that is. (I.E. the battery, the engine, doesn't matter)
See if noise is gone. If not, I'd try a different set of rca's just run straight from the amps to the radio without 'installing' them. Just run across the floor of the boat and see if a different brand works. If not, your ground loop is bad and may require an rca isolator. But... just a heads up if you use one though... many amps (most actually) use a ground at the rca for proper operation of the preamp. One more thing, if your gain on the amp is set to high that the head units output at 'clip' doesn't match (gain to high vs output capability of head unit) You can get the noise you are experiencing. I've been in this business since 1989 and own a car audio manufacturing company so I have a pretty tight grasp of this situation. Also... if you have a Pioneer head unit, it may have a 200 ohm rca ground which is a problem of its own. Why does it have it?? Because decades ago Pioneer realized most DIY installers do it wrong and have poor grounds. They built in resistance to help eliminate that noise you hear by defaulting a poor ground. Audio control did it too but gave the end user a switch from 200 ohms to zero in case their equipment was installed correctly. Anyway, good luck fixing it. Should not be difficult.
