I drove through high water and had to replace my engine. The water never was higher than the rocker panel, and now with the new (used) engine, it will run briefly, but not stay running. Could this be the ECM?

 

Could be. Did water get inside? There are computers and sensors all over the place, including under the carpet.

 

no water got in the car at all

 

Why does it quit running? No spark or no fuel? Shoot a little starting fluid into the throttle body and see if it runs a bit then dies. If that's the case, your fuel pump or fuel pump relay may be going out. If that's the case, I'd swap the relay with a known good one. I got a truck once the previous owner had replaced the fuel pump and it still gave trouble. Swapping the fuel pump relay with the A/C relay cured it. Swapped them back and put a new relay in.