Tom, Welcome Aboard !!
There are a couple of members here that are "Keys Enthusiasts".
my advice would be to look at other boats in your area and
see what they are using - ask around the marinas.
people "up here" don't fully understand the harsh salty elements
of the Keys. It is not just the salt, it is the finely ground up coral dust
that eats everything up in combination with the salt.
Like Rich said, some parts of the base and spindle may not be stainless.
(and like you have experienced with the seat mount).
rust stains on a nice finish is a pain (as you know).
do you have access to someone that does specialty welding in stainless and aluminum ??
have a couple made to your specs - one time and done.
post up some pics of the nice grouper and snapper you catch down there !