Were you planning on desalinating the seawater first? Because that requires an immense amount of energy to do, and we haven’t even touched on the topic of the energy required to move water from one place to another.

If you weren’t planning on desalinating the seawater, then the salt will definitely destroy whatever nutrients the desert sand might have had, rendering seawater irrigation completely pointless.

I remember somebody pointing out that one of the interesting projects that would become available with the development of Fusion energy would be large scale terraforming of deserts. The technology we essentially already have, the problem is that the current technology is incredibly energy inefficient, but if we don’t care about energy efficiency, because we essentially have more than we will ever know what to do with, we can just go ham.