I recently purchased a new (used) boat. It has a 350 merc and uses 2
batteries.
I think 1 is a starting battery and the other is a deep cycle w/
cranking amps. There is a switch for the batteries to select #1, #2,
or both. No manual.

I am not familiar with this setup.

1 - why is this system utilized (why not just 1 battery?) and what is
the proper operation of this system?
2 - should I always have it switched to "both" so both batteries get
charged when running? what is the preferred switch scenario?
3- which battery is better to run the electronics to? (I imagine the
deep cycle.)

Thanks for the advice.
-mark