I am ok with a canal but having a lake instead of a desert requires much more environmental consideration.
first of all, having a body of water means having a massive heat capacitor sitting in central desert. Meaning now the desert is not going to cool down at night. So the immediate result is less fluctuation in temperature which may be a good thing or not depending how you look at it.
The other thing that it may cause is high winds due to temperature differences. Winds that we didn't use to have, in desert it may mean sand storm or more floating particles in the air.
It will also make the whole dry weather inside Iran more humid which makes the heat more unbearable.
I wouldn't worry about underground water as you can always desalinate the water and use it instead of underground water and also can sell the sea salt which is pricey.
Having said that, central desert of Iran used to be a big lake long time ago. We also know it was not a sweet water lake as now we can see the salt spread around the desert that is the remnant of that lake. It may not be bad thing to bring it back but at the same time we need to realize it means a big environmental change with a lot of impacts not all of which may be welcome.