The MOB+ User Manual instructs that power to the MOB+ should be drawn from after a main power switch. This is to ensure that a stable power source is used, and that long stretches of wires using to small gauge (thickness) which can cause voltage drops, are avoided.
Also the user flow of the MOB+ where the system is powered on after the main switch will allow connection between the xFOB and xHUB before the ignition switch is turned on which is a more natural way to start a boating trip.
Snapshot from user manual showing power recommendations
With the above in mind, the power to the MOB+ can be drawn from other sources without any issues as long as the voltage source is not susceptible to voltage drops and the connection is done securely and water proof.