Plants require a ton of desalinated water and Animals who eat plants as such require desalinated water too.

There are countries in middle east like UAE, Saudi arabia etc. which rely on desalination but they are relying it for the clean drinking water, not for the food generation. They import almost 90% of their food iirc.

The amount of energy required to desalinate all water and the environmental impacts to get that energy would literally be quite catastrophic and I am not even sure if it would be even feasible and food prices would literally skyrocket or food would simply be produced even more less by magnitudes of order.