Hi there,
I offer water in a sippy cup at every sit-down meal, and started when she was around 6 months. Probably babies don't "need" water for a few months, but it makes sense to get them used to drinking it (mine hated water for the first couple of weeks) early so that they are used to it when they need it, and so that they know how to handle a cup.
Mine only drinks a couple of good swallows of water with each meal - then isn't interested, so I keep offering it and letting her try it out. I think the BM gives them all the hydration they need for the first year, and if you're worried about constipation, add pears to your cereal mixture in the morning or a tiny bit of prune juice mixed in, and that should do the trick.
Jana