I flew a few Hong Kong Express flights during the past few months.

You are constantly reminded that you are not allowed to bring outside food or drinks. I saw ground agent asking passengers to finish their bubble tea or coffee or throw them away before boarding. However they will not say much if you bring your own bottles/travel mugs for water. I see passengers bringing a small bottle of Evian onboard and the ground agent snd crews did not say anything.

About snacks, I have a small protein or oat bar with me, and as long as you don't eat them during service or trying to be discrete, most F/As will not say much.

However Hong Kong Express is a low cost airline so they rely on selling food & drinks for some additional revenue. They have a good array of pre-order meals and the prices are not out of the world. I will suggest you eating prior to flight or just pre-order some of their meals.

No they don't serve complimentary water.

Carfield