seawater for flushing, provided by WSD, is very well treated, WSD created the world standard in fact because none existed.
In the future maybe not, some smart greenie wants WSD to use toilet waste water for flushing good headline grabber - but forgetting enormous environmental impact due to chemical treatment residuals, probably dumped into the sea
seawater is highly corrosive, therefore all the components are plastic including the piping normally UPVC.
one other point, depending on where you live, the water pressure in Hong Kong is high, in NT pressure reducing valves are (illegally) fitted to stop the water heaters bursting, so do check the water pressure.