What would be the best metal for support and 24/7 water exposure in a water
feature?  Stainless rod?  Brass?  Bronze if it's made and available.

It's going to be used to support slabs of granite, none more than five
square feet by 3/4" thick, but the vast majority being one to two square
feet in area.

What would be the best welding process for each to get a lasting weld under
those conditions.  There will be no galvanic flow of current, save any that
may be created by minerals and the reaction of the various metals.

I may be able to get some copper rods from an electrician friend.  How do
you think they would age?

The entire structure would be heavy, and structures would use from six to
thirty square feet of granite 3/4" thick, plus the weight of the rods, plus
the weight of the water on top of the horizontal elements.  The design,
configuration, cross bracing and triangulation of stress points would be
critical.  Individual pieces of granite will mostly be oriented in a
horizontal position, using just three support rods.  But if I get into
anything fancier, more will be required to hole pieces in varying angular
positions.  All water features will be made to disassemble for cleaning and
to be transported to final location from fab shop, so there is no need to
make them to be hoisted, moved, or transported as one complete unit.
Therefore, I'm considering keeping the diameters small for aesthetic
purposes.

Steve