Air compresses, water does not. That's why when you compress the air the water collects in your storage tanks. I have seen large compressors that were not properly maintained where the storage tanks were half filled with water.
We had a warm humid winter this last year and we experienced water build up in the tanks of our practice robot for the first time ever. It wasn't a lot of water but I was never the less surprised by it.
If you are experiencing this because of your climate you should have a way to easily remove your tanks from the robot so they can be drained. Keep the inlet pointed up so the water can collect in the bottom.
The plastic tanks from AndyMark are great for this because they are held in by clips and are easily removable.
Generally most pneumatic components are designed to take a little bit of moisture. There is a reason that cylinders are mostly made form stainless steel.