No need disconnect the battery re the airbag. The airbag can be completely left in situ and alone so no need to disconnect battery.
On the A pillar trim there's a little plastic cover with 'airbag' on it - prise that out, which reveals a screw (torx I think but can't recall). Unscrew that, and then prise the top away using fingers. Once the top's away, lift it up and out for the bottom clear the dash.
But - before you do that, have a proper look under the car with a torch to see if you can find the drain point exits - look for small rubber flaps in the floorpan just behind the front wheel arch. If you can find them, it's easier to clear drains from the bottom up than the top down.