I also had issue regarding this matter when i traveled from Hk. I did two long hour flights with my daughter, to LA when she was 4 month and to Sydney when she was 6 month. The security check spot after immigration is very easy to pass if you explain to them what that water is for, but if you travel to USA or Australia, there's another security check in front of gate and they usually don't allow you to bring any kind of liquid. Airport staffs and security staffs in HK airport seems like they don't have very clear guideline because they let me pass the second security spot without any problem once and then made me pour out the water another time. Actually I'm very upset with this. An airport staff explained to me that this is US and Australian goverment's requirement not Hong Kong's. But when I checked with the airport security staff in the states and Australian goverment's official website, they have no problem with moms bringing steralized water to feed their babies.
No airline can make boiled water on board, they just serve 'hot water' from the tap and if you travel cathay pacific in economy class, good luck for us. They even don't serve mineral water to economy class passengers. It's 'tap water'!
I was upset when I checked HK airport website and found the quote that they allow passenger to bring baby food but not water because it can be provided on board. They serve special meals to people who have restriction on their food because of religion or health condition. Then aren't they supposed to provide some pre-boiled water to the passengers with baby if they request?