Which one is my starting battery?

[MJHSupra]

I'll use 1 or 2 to start the boat, then switch to BOTH while under power. This charges both. Then if we stop to play and swim, I'll switch to one battery or the other, to ensure we have a full battery to start the boat.

Sounds right on the new boats. I think the advice from SC is to operate the boat in #1&2 (when running) and have it switched to #1 or #2 when not operating.

Thus, if you are starting, you could use #1 position, #2 position, or #1&2 position - just do not switch of OFF when the boat is running.

For the charging and the alternator:
#1 position and the alternator only charges that battery
#2 position and the alternator only charges that battery
#1&2 position and the alternator charges both batteries