Aqua Chem already does that for a living. They make all sizes
Musk should learn how to do water desalination on the cheap.
He has all the tools.
All he needs is to design desalination plants sized to fit on a space vehicle, truck or barge. Each plant could stand alone or snap together with other desalination plants of the same design to form larger plants.
Each plant would be created using the stamping process he uses with his car. He could scale the process by using the same stamping machines to create similar size HVACs.
the lowest cost desalination plants are made by israel and singapore. they make desalinated water for about $500@acre foot. Musk could do the same job for $300@acre foot. At that cost desalinated water would be cheap enough for all agriculture except for field crops.
Aqua Chem already does that for a living. They make all sizes