I have travelled out of Hkg to many countries (U.S, Australia,Europe,Asia) with my daughter and I usually carry a flask of hot water and I filled the baby bottles with boiled water. So far I haven't had any problems with security in Hkg at all each time I travel. For flights to U.S or Australia, they have another security check at the airbridge just before you board the plane, but that is not a problem as well for me with carrying the water. My guess is as long as the water is in the baby bottles,and they see that you are travelling with a baby,then it should be fine.