I have to write with film scores (pretty much anything by Thomas Newman, with occasional appearances by Alexandre Desplat)/instrumental music (i.e., various entries in the Final Fantasy video game series, or any of those 8-hour-long, coffeehouse-type-ambiance compilations you see on YouTube, etc.) in the background—it helps get me into the zone, so to speak, increases my focus. But if there are words in the music, forget it—all my concentration goes immediately to the words!
So, music is a given, and while I usually have my water bottle nearby, I'm often so focused on the writing that I don't even take any sips from it!