IF the mechanic can start the car from underneath, but you can't use the ignition/start switch, yes it is probably the switch and/or the wiring to and from the switch.

That should be checked first with a multi meter. If those check out, you may have a problem in other wiring/modules. Don't forget that there are a number of ignition "interlocks" that make the car not start in gear and/or without the brake pressed - that would be the next thing I checked.

I agree with @yaser there was most likely damage done to the wiring from the ignition going to the starter. There is a lot of wiring involved so its not going to be easy. You can start by finding the wiring diagram for the ignition/starter circuit. Also check the fuse for the starter or starter signal.

I drive a 2017 Buick Verano. One day my street was flooded and I vaporlocked my engine. The water wasn’t very high. I image the water sucked through the exhaust. I have replaced the engine, the starter, the battery, and the mega fuse for the battery but the car still won’t start from the ignition. The mechanic can start the car from underneath the vehicle but not at the ignition. Do you think it’s the ignition switch or something else? 

 Thanks Willie.

if the mechanic was able to start the car directly from the starter, you should start with checking the wires from ignition switch to the starter.