[temporary22]

Salt kills most plants. Bringing it inland would basically stop agriculture for hundreds of meters surrounding the canal. If it goes into the aquifer it's even worse.

Also, inland is usually higher than sea level (where it's lower you get lakes / ponds / swamps, unless it's actively managed), so you would be creating ditches a hundred meters deep if not more.

I'm also not sure that local water would increase rainfall, any amount of wind would sweep moisture off immediately. The most it can do is to increase the humidity of the surrounding area when windless.