If Hong Kong seceded, China would have no responsibility what so ever for the island. Let the Hong Kongers beg for water.
Besides, a total water cut off wouldn't be necessary. Beijing could reduce Hong Kong's ration of fresh water down to where everybody on the island could still live but not take showers. Some Hong Kongers like to think mainlanders are dirty and smelly, but after a few weeks of not showering, Hong Kongers would be really dirty and smelly.
Secessionists hope to improve Hong Kong's situation, however unlikely that hope is. I am saying that Beijing has enormous scope to make life in independent Hong Kong much, much worse -- just as the Ukraine is now far worse than before its 2014 revolution, in spite of many promises by rebels.
After secession, if it happened, Hong Kongers would no longer be citizens of China. So why should Beijing care for them?