Ferry infrastructure from HK Airport is just too good to justify direct service to SZX or CAN
To Shenzhen: Hop off HK flight, walk 100 yards to ferry, leaves hourly, $30 in First Class, 30 minute ride
To Guangzhou: Hop off HK flight, walk 100 yards to ferry, leaves hourly, $60 in First Class, 90 minute ride
The original cities discussed on this thread (BKK, SGN, MNL, DEL, BOM, BLR) all seem like a better use of an ULH frame from SFO.