Does rust cause more rust to a nail then water?

-->Yes, in most cases. This poses, however, strong arguements both ways.
Think of the rust as a seed--where the O2 finds the host ion molecular structure(salt, acid) often concentrated to an area in which to assist a Red-Ox reaction on the Fe(Iron) content of the steel-->Fe Ni Mn .
However, in any assessment-"Quantitative" or with simple "Logical-Assumptions".. your answer remains completely "Dependent" upon the "Margin of Uncertainty".
Like for instance, assuming the material is poor-quality steel, a "rust seed" consisting of a drop of salt water(wet/dry) on the bare nail surface would differ drastically from a vinyl or coated nail having a clean drop of distilled water present.
In your case it is likely that the present rust will be a contaminant concentrated in the rust and therefore remains much more rusting to go.
Ok, hope that helped! P.S. --> Both "Rust" and "Steel" are good key words for Wikipedia searches of this kind. Which I basically rewrote the page there. Included below is not mine except for the end...

-MajorUtah.. (*Xy'

Reference material-->
Wiki-->
Galvanized- treated for resistance to corrosion and/or weather exposure

:* Electrogalvanized - provides a smooth finish with some corrosion resistance

• Mechanically galvanized - deposits more zinc than electrogalvanizing for increased corrosion resistance
• Hot-dip galvanized - provides a rough finish that deposits more zinc than other methods, resulting in very high corrosion resistance that is suitable for some acidic and treated lumber; often easier to bend than other types of nails,
  
• (MajorUtah) -->and from my experience, I'll assure you that in the long run Galvanized, though intrinsically designed soft to frustrate worker, proves superior in both corrosion (Red-Ox) and over all slip or "work out" time.. eg; a vinyl coated nail has great corrosion resistance, until your hammer strikes the head and exposes the bare material} { and a rough galvanized finnish will firmly grab the surface of the wood..--