Even if it were possible to convert seawater to fresh for free, with no downside to the environment or other collateral damage, there would still be problems. It will take energy to pump that water from sea level up to the farms and cities where it is needed.

Potential energy U due to the effects of a gravity well of depth h near the earth's surface is

delta U = M g delta h

where U is in joules, M in kg, the gravitational constant g is 9.8 m/s**2 and h is in m

a perfectly efficient pump capable of lifting a kg of water 1 meter every second would consume 9.8 Watts.

So, approximating in English units, if your farm or city is 300 feet above see level and it requires a thousand cubic yards of water per second, it will need a power source capable of generating about a Megawatt.

That's not unreasonable, we routinely pump water from deep wells all the time. But it is not free.