That is a very good idea to conserve water. Water is such an essential part of environment and we are wasting too much. If this waste continues, it won't be long when most of us will face shortage even for basic necessities. The places that are close to ocean must use this natural resource .
If it can be done without polluting the seawater, California should use seawater for toilet flushing. Water is becoming a scarcer and scarcer resource, and with a ready supply of seawater close at hand, California could alleviate water shortages somewhat by using this resource. The water does not touch people and would not be used for drinking, so it is a safe resource and should be used.
I think many regions of the world should adopt this policy, including California. Not only is fresh water becoming scarce but as time goes on the number of people without access to safe water still remains at unacceptable level. Seawater is abundant and its use in toilets saves freshwater so there really is no downside.
When resources are scarce everything possible has to be done to look after them, it is not an option it's a requirement. The fact that Hong Kong have a functioning system should lead the way for other regions like California to also adopt the policy. California could learn from Hong Kong and get help with implementing it which adds weight to why they should adopt the policy.