Shale gas in Sichuan is a terrible idea, for gas production and water usage... There are plethora of reasons I won't go into.
But desalination of sea water for coastal usages is not economically feasible. It is extremely expensive and environmental damaging. The amount needed by even just the coastal provinces would require more energy than China is current producing with change. Also transporting it further inland would also be expensive. A single oil pipeline is in the order of billions, now water requirements are 100x more, so expect to cost even more (not linear of course).
The latest desalinisation plants have a cost of $0.3 per m3 of water.
If the average person uses 150litres per day, that works out as $16 per person per year.
That is affordable.