When shopping for a ECU, make sure it's compatible with your car. There are lots of ECU's out there, and only the one that matches your car will fully work.
Year: 03-04 (drive by cable), 05-08 (drive by wire), 09-10 (new style)
Transmissions: Manual, Automatic
Drive wheels: AWD, FWD
Engine: 1ZZ, 2ZZ
Edit: Matrix and Corolla ECU's should also work for your car, but they have to match your equipment.
*Sold* 06' Vibe Base - Mono Platinum - Manual
Turo'ing 15' BMW 328i Sport - Jet Black - Auto
Daily 19' Toyota Camry SE Hybrid - Galactic Aqua Mica - eCVT
In that situation the cars computers are the last thing to worry about. They are well hidden in the dash.
If the ECU or other computers get wet, the rest of the car is a write off anyway. Also, they are easy enough to come by on ebay or car-part.com or local wrecking yard if it does come down to that.
If your car gets submerged high enough to short the electronics, then there would be water everywhere. Crank case, transmission, gas tank, intake, exhaust, sensors. Quickly swapping out the ECU in an emergency is not going to get the car running again.
In the case of Katrina/Sandy Shelter, Water, and Food should be first on your list. Next would be Gasoline. After big events the power usually goes out so gas stations can't use the pumps to get gasoline.
*Sold* 06' Vibe Base - Mono Platinum - Manual
Turo'ing 15' BMW 328i Sport - Jet Black - Auto
Daily 19' Toyota Camry SE Hybrid - Galactic Aqua Mica - eCVT
Also, if you are thinking of saving the car in the event of an EMP, there are so many things that will be affected, you would be better off putting the whole car in a giant electronics bag. Lol