Most land based plants would die if fed saltwater.

The water would in most cases sink away below the surface too.

First off, the water would need to be desalinated or you would ensure the land would be unsuitable for farming (and really growing anything) for generations.

Also, sand doesn't hold water. In fact, when planting trees and other bushes, if you want more drainage, you typically add rocks and sand.

Second, most plants need non-sandy soil to grow on (palm trees and other beach bushes and plants aside) though those grow in areas that have lots of rain already.

Thirdly, the soil will need bacteria to aid the plants in obtaining nutrients and breaking down waste (dead leaves, dead plantlife, etc).

The way to do it is to look at a couple of projects that are fighting against desertification in Africa:

1. The Great Green Wall https://education.nationalgeographic.org/resource/great-green-wall/

2. Using compostable waste to fertilize soil https://jstories.media/article/greening-the-desert-with-trash

You'll notice that many of these projects start at the edges of deserts. Instead of relying on pumping water onto sandy soil (which would just suck up the water as sand doesn't hold water that well) they focus on extending the non desert ecosystem onto the desert so that the new soil will absorb water better, the weather over the newly terraformed area will be less dry, and it will eventually be self sustaining.

Saltwater doesn't revive soil, it kills it.

It's been done before (though not intentionally at first), at the Salton Sea.

And the results weren't that bad (granted possibly a smaller scale than some are imagining when they forecast doom).

https://www.ppic.org/blog/the-troubled-history-and-uncertain-future-of-the-salton-sea/

But the problem is any deserts are deserts for a reason: lack of rainfall and/or natural inflow from rainwater upstream. The result is that you have to keep pumping in tons of water and/or rely on agricultural runoff which is nutrient-depleted and usually full of chemicals.

Read the rest of the linked article for what's going on with that one.

Deserts are actually very important to the worlds ecosystem. The Amazon rainforest probably wouldnt exist without the Sahara. A lot of the Sahara's sand is made up of dead plankton and these dead plankton are blown across the seas by strong winds and eventually land in the amazon. These plankton give the soil extra nutrients and are one of the main reasons why the south Americans are so lush with life.

The sheer volume of water that moves through even a small creek is shockingly massive.

The amount of water held in an aquifer is astounding.

The soils required for agriculture and general growing plants (ones that hold water and nutriet) specifically are lacking or depleted in deserts.

So... it's a literal pipe dream.

Have you ever heard the phrase “salt the earth”? That’s the fastest way to kill everything in the soil and make sure nothing grows for a very very long time.

Most deserts have life, which you would kill. But for sandy deserts, have you never built a sand castle?

The reasons are legion, starting with the fact that it wouldn't work.

Because that's how you make salt lakes. Instead of reviving them, it would kill off the last remaining traces of life.

In addition to what everyone else already said, it takes a tremendous amount of energy to pump water uphill.

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.

Salt.

If the area’s enviroment didn’t have it to start with adding it will kill off whatever lives there.

Also? Generally there’s a reason it’s a desert (lack of rainfall,) and is itself (usually) a vibrant ecosystem.

Saltwater destroys soil and vegetation instead of facilitating its existence. And you’d need to add better soil and the means to produce more fertile soil (plant species that shed leaves often and have nitrogen-fixing microbes in their roots).

But such water would make rain in the surrounding area more likely and common, if it can be sustained. For instance, I hear there are plans to re-create an inlet in northern Libya that used to exist but dried up when it was cut off from the Mediterranean by an earthquake that pushed up a natural dam some 6000 years ago or something, so the surroundings can become greener. (But given the current flooding of roughly that same area, doing so would be a terrible idea for the people who now live in that below-sea-level area.)

Better yet, use Brawndo!

A lot of deserts get a fresh water pulse occasionally, and things spring back to life. If you take that away it destroys more ecology. You're better off making multilevel greenspace in the cities.

If the ice caps keep melting, we're going to see what happens! In real time

History teaches us that when we mess with the ecosystem, something dies/extinct

Aside from the issues stated already. This would need to be a truly monstrous amount of water. The cost would simply be far too much to justify any sort of benefit.

Project plowshare 2.0

Why would you want to?

I was thinking that a solution to rising sea levels could be a canal from the Atlantic to that part of the Sahara that’s below sea level.