The least expensive means to control the volume of an amp, besides the head-unit fade, is to use a line level POT. But by time you go with 2 so you can control 2 amps, plus the 4 long RCA pairs, you are approaching the cost of an actual marine zone controller like the Kicker ZXM-RLC, which is also a 9v p/chnl line-driver.
ThrottleBox Remote Controller
Allows the user to actively shape the sound of HLCD speakers depending on the real world listener position i.e. wake boarding (far out), wake surfing (close in) or cove (stationary)