On Wednesday, September 4, 2019 at 2:15:42 PM UTC-4, micky wrote:
My brother lives on a barrier island** and 3 or 4 times a year, there is
a king tide that floods not only the beach but the main north/south
street. In many parts of the island, there are no parallel streets with
which to bypass the flooded one (and come to think, all of them would be
flooded too.)

One time years ago he tried to drive through and his recollection is
that the car stalled right away and wouldn't start, and the shop said he
needed a new alternator, and after they replaced that things worked.

Shouldn't the battery have been enough to power the car for a day or so?

A day? Maybe 30 minutes, if you're lucky and the battery is in
reasonable shape, depending on what loads are on, eg headlights, etc.





Or is it possible with salt-water to short the alternator so that the
battery won't run the car at all?

I've seen videos of cars on fire from trying to drive through salt water.
It can short the alternator or anything else that's available. I guess
you're brother learned an expensive lesson.