The only concern is them getting rid of the milk bottle I think it's the new texture and the new sensation of cold water that they enjoy. Its new to the baby. Just try and encourage them to go back ti the bottle soon as they drink water possibly wait 20 mins hide the water then finish the milk
Don't worry. Let em.have water they won't fill themselves up as it has zero calories and goes quickly. They will feed better of their hydrated. I allowed my baby water she loves eating and drinking xx