I don't get the whole bringing water on board either. I have a 32oz stainless steel water bottle I use. The only thing I do bring is those little single serv powdered drink mixes. (Kool-aid, Hawaian punch, lemonade, green tea etc) I like a little more choice then the free lemonade or tea on board.
Of course I do buy the fountain club card to spoil myself with soda while on board.