OpenMarine

I like the idea of the power module protecting the system by allowing for an orderly shut-down, but I don't like having the unit, or anything except the bilge pump, permanently attached to power.

To be honest, it's more about me using the master breaker to shut things down.

Anyway, could a small 12v battery dedicated to the pi, or a capacitor, be the answer? Something that would charge up with power on and then supply enough at power down to allow the orderly shut off.

What battery, or capacitor, would reasonably be suited to this purpose?

A small 12V battery to power the Raspberry Pi during shut-down can work. The smallest sealed lead-acid pack should be plenty to run the Pi for the few seconds it takes to shut down. The Pi will draw a maximum of 2A @12V.

But you will need to switch this leg separately from the other 12V consumers to avoid back-powering when the master breaker is shut off. This could be done via a multi-pole switch as master breaker, or a 12V relay to control the 12V leg to the Pi, or a beefy diode.