Cormet,
You'll not find in CorSur or other DataBase isobutane+water. To know the corrosion rate of your fluid, is necessary to understand where this water come from, and the type of systems (is it a tank, a pipe, a can?).
Water, mainly, is corrosive if it contains dissolved O2 or H+, acids. If you have a closed systems, with water with a certain amount of O2 dissolved at pH near neutral, you will have corrosion until all the O2 is consumed by the corrosion process. Once the O2 is consumed you 'll not have any corrosion unless some O2 can enter the system and dissolve in water.
In my opinion if you want to assess the potential corrosivity of your system you have to define:
-type of item (tank,pipe...)?
-is it a closed system or open in contact with air? it will be stagnant water?
-what type of water you'll have; formation water, condensing from a gas...., and the chemistry (chloride?)
without all this information is not possible to say something.
regards
Strider