Very sorry to hear about your problem. Flooding and electronics is very bad news.

Electrical connectors that were covered in water are more likely to corrode and create problems now and in the future. Some areas could still be holding water. I would go first to the "Security System Module / Central Locking Pump - under trunk padding on right side". Others have had problems with trunk leaks and water getting in that area. Water shorting electrical circuits will cause low voltage and problems for other areas.

The good news it sounds as if the water did not get inside in the passenger side of the floor, under the dash, or in the most important control module box next to the battery. There is a possibility of getting it going again.

It would be very worth your time to check all of the fuses. If you find any blown check the devices they protect for water.