"Non, tu bois l'eau."
Here it's "Non, tu bois l'eau" so the negative form would be "Non, tu ne bois pas l'eau". There isn't always "de" after a negative form! "Tu bois l'eau" probably refers to a specific glass of water here. Saying "tu ne bois pas d'eau" would mean the person shouldn't drink water at all, whether it in the glass or not.