yes, water - and as long as it is wet, you can also work it with a brush. As soon as it dries, it is pretty permanent. That's when I do additional fine-tuning with hard erasers and also with electric erasers.
What does not work however is charcoal powder with water - it does not stay on. You can even blow it away when it dries. But somehow, the charcoal sticks work good. I use Kooh-I-Noor, the Gioconda round sticks in a holder, and also thick flat sticks .
This is one panel where I used water, sort of abstract, with my fingers, erasers, sandpaper etc: https://www.flickr.com/photos/artofthemystic/16370912900/in/album-72157613411164741/