At that point, Hong Kong was entirely dependent from the fresh water provided by the mainland, to the point that if the taps were ever closed, hundreds of thous

At that point, Hong Kong was entirely dependent from the fresh water provided by the mainland, to the point that if the taps were ever closed, hundreds of thousands would have died before alternative sources would have been available. You cannot just ship enough water for 6.5M people, and HK authorities were scared that if they started constructing desalinization systems, PRC would pressure them to stop doing that with the threat of immediately cutting off the water. PRC had effective power over HK before they were granted legal power over it. Before the time of the transition they stressed that all the current treaties for water were written to end at the time of transition, and that they would not be willing to renegotiate them. Any attempt to keep HK or to turn them over to Taiwan would have resulted in mass death and evacuations.