Is it true that iron (Fe 0+) cannot exist when there's water around? So somebody told me that iron metal (zero charge) cannot exist at all when $\ce{H2O}$ is present because it will all turn into rust.
How is this possible? Isn't there some sort of equilibrium?
Does it have to do with the pH/pE? And if iron really can't exist, then what happens if I put an excess of iron into the water?
Iron metal can exist in presence of water. As you stated, important point is the $\ce{Fe^0 -&gt; Fe^3+}$ reaction. Water is not an oxidizing agent, therefore is not capable of inducing this process.

To create rust, you need also oxygen, or as you noted, electrochemical potential.