Try offereing a sippa cup with a straw. Sounds silly for such a young age I know, but babies natural instinct is to suck. they might play with it for a bit but once they suck and figure it out they get it pretty quick. Ashlea was having her milk out of straw sippa cups by 7 months, and water in them before 6 months. She would get angry at the bottle cause it was too slow.
We also would use them at the day care for the little babies and so many of them used them really well. My director also told us that the straw sippa cups are really good for developing certain muscles in their face that help them speak. The best thing was you can get them for a couple of dollars at the supermarket.
Like anything, the more you offer it, the quicker they will pick it up. Its also better for little babies to be offered smaller ammounts of water more often, than for them to drink alot in one dose.
The younger you start offering them water, the more likely they will drink it when they are older.