If you are using this for surface supplied, and you have a power source on board (12 volt DC, 120 volt AC),
you could use a barkesdale or similar pressure switch monitoring the pressure between the ouput of the
compressor and the unloader valve (load genie). When the valve shifts and idles the compressor, the pressure in that
leg of piping gets vented to air. So if you set the switch to 50 psi or so, when the unloader was sending air to the
tank, the switch would be closed. When the unloader shifts to idle the engine and vent the output from the compressor,
the switch would open. Then you could tie this switch into a solenoid valve that operates with whatever power source
you have on your boat (12 volt DC, 120 volt AC, etc). It also helps to have a pressure guage monitoring this leg of piping
so you know what to set the switch to. Remember this piping gets pretty hot and has lots of vibration, so plan accordingly
(glycerin filled guage, use brass vice plastic body switches, etc). I've done this before for a couple commercial diving (harvest) applications.