My DD had her first sip of water probably before she even a month old (please, no judgements) she is BF with a formula top up before bed but gets hiccups pretty often after a feed and if I leave her with them she gets upset and throws up. I found trying to BF her more to get rid of them would just result in power chucks but a mouthful of boiled cooled water would get rid of them without overfilling her, that and we live in the tropics without a/c and she's a very warm bub so I figured it wouldn't hurt, and it hasn't. so she likes a bit of water (about 20-30ml) every few days but it in no way affects her feeding. I think the main concern with giving bubs water is that it can fill them up and they might not get all the milk they need, so maybe if you try giving them water 10 mins after a feed?

Just my opinion and what works for me, I'm only new at this mum job!!!