Will an iron nail rust in salt...

The corrosion of iron indicates a chemical change in the metal. Rust (hydrous oxide) is an example of this change that results when iron is exposed to water or damp air. A thin film of oxide forms on the iron; this actually protects the metal from further corrosion by slowing the rate of oxidation. Where salt is present, electrochemical corrosion occurs, and the protective oxide film does not form, thus the corrosion (buildup of rust) continues unchecked. Your iron nail will indeed rust more quickly and severely in salt water.