Water for sure for me. I really like to taste unadulterated tobacco, I find it's hard enough with all the other variables that conspire to give me inconsistent smokes(packing, lighting, the pipe in question, star alignment, etc). Although I am a very big fan of elevating my spirits prior to a pipe, with an adult beverage.
There's also something to be said for the classic morning coffee and morning pipe combination-- they pair too well to be neglected.