On Feb 25, 5:49 pm, wrote:
I am wiring my boat with a Perko 1/2/ALL switch and I'm connecting the
hot (+) lead from my accessory panel to the COMMON termial of the
switch (this is the same terminal that runs to the engine as well).

My question is this: Does it matter which battery I connect the
ground (-) to. Seems like a dumb question, I guess, since both
battery grounds are conncected together, but I thought I'd ask anyway
just to be sure.

Many thanks...

Your batteries are connected in series per your description. You can
ground your switch on either battery or any other ground you can find.
If your accessory box is grounded to your engine, go with that ground
since its more convenient.