I haven't seen the GM test where the water came up to the windows (do you have a link?).
The standard water fording / water trough test the Volt went through seems to have about 1 foot of water at the deepest (the deepest test had water up to less than the center of the wheel hubs, which are about 13 inches tall, given it has P215/55R/17 tires).
http://www.greencarreports.com/news...ars-make-you-worry-about-toasters-in-bathtubs
The water chamber test done on the Volt is just a standard leak test to simulate rain (not the same as standing water):
http://www.gm-trucks.com/news/newmo...-takes-a-bath-in-gms-universal-water-chamber/
Tesla does a similar leak test on the Model S with ultrasonic waves (to save water):
http://www.teslamotors.com/blog/tesla-factory-birthplace-model-s
The Volt's warranty will not cover damage to the vehicle from water or fluids either (page 11):
http://www.chevrolet.com/content/dam/Chevrolet/northamerica/usa/nscwebsite/en/Home/Ownership/Manuals and Videos/02_pdf/2011_chevrolet_volt_warranty.pdf
Although in real life, it seems they are willing to make exceptions to the warranty:
http://gm-volt.com/forum/archive/index.php/t-7892.html
From the same thread there's a claim that a Prius under water had to get its battery replaced.
In any case I would not recommend going through deep water with any car, if it can be avoided. There have been warranty claim issues even in less than a foot deep water, an example is this Jeep which went through 10 inches of water, got hydrolocked and Jeep refused to cover it under warranty despite advertising up to 30 inches of water fording at slow speed, since changed to 19 inches:
http://jalopnik.com/5697449/why-did-a-new-jeep-wrangler-die-in-ten-inches-of-water
I doubt most car companies will see 1-2 feet of water as a reasonable depth for a sedan, given even a trail rated SUV like the Jeep only covers about 1.5 ft of water fording.