I always have protein bars and perhaps some nuts with me for healthy snacks and, as others have stated, to be self-sufficient in the event of prolonged delays. I always bring a bottle of water if I'm in Y but usually don't bother if I'm in F or C. (Particularly with delays arising from being held on the ground, the first drink service in Y can often take an hour or even 90 minutes from boarding, and that's a long time to go without water, especially on a plane.)