Alternator already relatively waterproof.

ECUs won't get damaged by short crossings. It's the car being stuck in a flood that's an issue. Most engine-bay electronics have their own rubber seals and will survive short trips.

The Ondoy damage to most SUVs was because the water seeped into the cabin and shorted out the less-heavy duty wiring inside. If your SUV has the ECU in the cabin, it isn't as well protected as one in the engine bay.