The American Academy of Pediatrics states that small amounts of water can be offered at around 6 months as long as your babe is exhibiting healthy and steady growth.
Offering small sips of water from an open cup can help with their drinking skills. Water is a tricky liquid for babies to get a handle on at first since it's so thin so you might notice a little coughing at first. Because your mini is getting lots of water from breastmilk or formula it's not necessary to add more than a few sips of water a day before 12 months.
Your little one should have 4-6 very wet diapers a day so if you're concerned that your baby is getting enough hydration contact your pediatrician.