Offering water before 6 months is not necessary and could be dangerous. 

Exclusively breastfed chestfeeding babies receive all the fluid they need from human milk and it is not necessary to offer additional water since human milk contains 88% water in its composition. In the days after birth, colostrum provides the nutrients and hydration necessary for the baby, as long as he is breastfeeding chestfeeding effectively.

Likewise, babies under six months of age,fed with infant formula also do not need to drink additional water to the milk offered since potable water is used for preparation, and this will be received during feeding.

Even on hot days or in cases of dehydration due to illness, it is not necessary to offer them water as long as they have human milk or infant formula available to drink.

It is also not necessary to offer babies water to "clean their tongues."

On the other hand, substituting milk feedings to offer water to a baby under 6 months interferes with the normal frequency of breastfeeding or chestfeeding. In addition, since the baby's kidneys are still immature, supplementing with water or glucose increases the risk of:
• Jaundice (high level of bilirubin in the blood)
• Excessive weight loss
• Intoxication

After six months, water can be offered in quantities no greater than 4oz-8oz, although milk remains the main source for hydration.

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References: 

Choose Water for Healthy Hydration - HealthyChildren.org