I always bring a bottle of water on board, mostly because I get thirsty (especially on long-haul flights) and don't want to spend the time or effort to get a FA's attention just to take a sip.

I do, however, try to save my own bottle, and drink what the FAs offer, as long as I see that it is bottled water. Sometimes later in the flight, I see they've switched to water from a pitcher, at which point I politely decline.