I think with high temperature, energy intense cold fusion methods this could be done.
By "high temperature" I mean distillation methods.
There are two ways to do desalination: reverse osmosis and distillation. Reverse osmosis takes less energy. I think it is more common these days.
I do not know the engineering details, but my guess is that with cold fusion we could afford to use both methods, in stages. First reverse osmosis, and then distilling the output from that. The goal would be to produce water as pure as rain, with as low salt content. Maybe some sort of filtering technique through sand or soil would also be needed. I wouldn't know. The point is, we could use extravagant amounts of energy, with methods that would be uneconomical today.
Rainfall from hurricanes near the ocean sometimes has a lot of salt in it. In the Inland Sea in Japan, rain mixed with seawater sometimes kills of patches of bamboo a good distance inland.
Some experts are saying we should concentrate on cleaning up wastewater and reusing it, rather than desalinating ocean water. That is what they are doing in Los Angeles. That sounds like a good idea, but you could not do that to irrigate vast areas of the Sahara and Gobi deserts, which is what I propose to do.