A great example of water being erased occurrs on page 18 when Oskar is talking about how he writes everything down, and states, "instead of singing in the shower I would write out the lyrics of my favorite songs, the ink would turn to water blue or red or green, and the music would run down my legs." I also see great imagery in this passage when Oskar talks about the colors the ink turns and how it "runs down his legs."
Writing was also "erased" when the man and the woman are communicating through the book. "[...]so I ripped the page from the book--'I don't speak. I'm sorry.'--and used it to dry her cheeks, my explanation and apology ran down her face like mascara [...]" (pg. 31) The words are again erased by water, but Oskar does not just see them as words, but more of his life.
I think that this passage shows that Oskar is, in a way expressing a sense of sorrow. The water falling from shower can also be symbolic of the tears he has shed for the loss of his father which often leaves him with a reoccurring sense of a loss for words, and gives him what could easily become misunderstood communication.
On page 75-76 where there is a letter from an unknown person from a Turkish work camp much if the writing is censored and replaced with capitols X's. This gives the reader a sense of the unknown and draws the reader in because they want to know who the person is and why the writing was censored
"What if the water that came out of the shower was treated with a chemical that responded to a combination of things, like your heart-beat, and your body treatment, and your brain waves, so that your skin changed color according to your mood? If you were extremely excited your skin would turn green, and if you were angry you'd turn red, obviously, and if you felt like shiitake you'd turn brown, and if you were blue you'd turn blue." (163) This quote shows Oskar inventing something that would help him see other people's feelings and other people to see his feelings. He turns something that's unseen to something that you can see.
I think writing being erased is symbolizing Oskar's fear of his father's memory being erased. This is one of Oskar's fears, because he wants to be able to remember his father and find out more about him even after his untimely death.
I think one of the things that's really interesting about Oskar's grandpa is that is that he doesn't erase anything from his books. He either goes on to the next page or crosses out a few words and modifies the phrase to better suit the situation. To me this goes into how he doesn't want to let go of the past and he wants to be able to remember and see what happens in his past, because he lost his entire past with the Dresden bombing, and now he can document it and it will never leave.