There isn’t anything prescribed in the Sunnah! One can drink whenever he wishes! Whether it is healthy or not, this is for the doctors to decide.

Is it not good to drink water after meals? The only reference I got after searching is that Allamah Ibnul Qayyim رحمه الله has mentioned that the habit of Nabi Salallahu Alayhi Wasallam was that of not drinking immediately after eating as this is harmful to the digestion process.