Most aftermarket head units (including marine ones) have a standardized color code for the wiring.
Yellow is the constant on battery 12V
Red is the ignition 12V that should only be on when the ignition is on
Blue is for a power antenna.. it sends a 12v trigger signal only when the radio is on
Blue/white is a remote amp turn on, similar to the power antenna, except it sends a 12v trigger signal whenever the head unit is on
Orange is for dash illumination/dimmer. If your head unit has it, it'll dim the lights on the head unit when you turn on the headlights. Don't hook that one up.
If the head unit only has a blue (and no blue/white) than that is for the remote amp turn on, instead of only for a power antenna.
Here's a diagram I found for all the wiring color codes for all the speakers, etc.