Car consistently dying even after battery replacements

Late December we had a flood that saw maybe
an inch of salt water in just my passenger side
floors. For about a week the radio and AC units
would flicker on and off. Eventually stopped
flickering on and off and both started to
consistently work again. Fast forward 2 more
months, I took the car on a long journey (1,090
miles) and everything worked fine the whole way
through. About 3 weeks living in Missouri now,
my radio turned off, my ac stopped working and
then my car died. I would jump it, it would work
for 1 day, only to die again. I repeated this
process twice before i brought it into the
mechanic and they told me my battery and
alternator were fine. I brought the car home and
then boom a day later it had died again. So i
brought it back to the mechanic, had them
replace the battery and i went on my way. Boom
the car dies again and again and again. Now im
left with no choice but to drive 2 hours to the
closest VW dealership and get it looked at. They
tell me german cars absolutely do not like after
market batterys and that they recommend i get
my batter switched (AGAIN) to a Volkswagen
specific battery and my problem should be
fixed. I get the battery switched and 3 days later,
the car is now dead again. Does anyone have
any clue on whats going on? Did the minor salt
water flood months ago finally come back to
bite me? Are the ac and radio drawing some
sort of power that nobody can tell? Anything
helps, thanks.