I don't want to stand there for several hours holding it up. And I don't want to pound nails in it
to brace it with 1X4's
You are over thinking this. If you mix the concrete up to about the
consistency of cookie dough and dump it in the hole in bug chunks, the
water will come right out and the stuff at the bottom will be plenty
stiff enough to hold up the post without bracing.
Pouring it in water, while it is dry is the worst thing you could do
because the aggregate will fall out of the cement that is lighter. I
still think for just setting a mailbox post it would be OK.
A don't even bother with concrete for my mailbox. I just backfill with
dirt and tamp it in. That way replacing it when it gets hit is not so
tough.
Maybe Harry will drop you off an empty kitty litter bucket. Those are
just right for this sort of thing since you can more easily hit an 8"
hole with a slug of concrete.
If you really want to get pedantic about this, how long did it take
for the ground water to fill the hole. If you dropped a bilge pump in
there and pumped it out, could you get the post in there and get a
foot of concrete in there before it filled back up?
The water was filling as I was digging. I wouldn't dump the mix on top of the water. I still like
the idea of mixing the sacrete in a wheel barrow, and dropping it into the plastic bag with the post
inside. No extra water to worry with.
The problem is there will be no "bite" with the dirt (lots of voids)
so you might as well just skip the concrete and tamp dirt around the
post. Trust me if you mix that concrete at about a 3 slump, it will
land at the bottom of that hole mostly intact.