Along with your sense of smell, your sense of touch is important to sensing moisture in a used car. Water from flooding tends to collect in locations even the dealer may miss.
Run your hands along the carpet and pat it in different spots to try to locate moisture. If you're able, peel back the carpet to see if you can feel moisture between the carpet and the car body. You may also see other signs of water damage, such as rust, by looking under the carpet.
Be sure to open the trunk and feel around the carpet there, as well. Then take out the spare tire and feel the material underneath. This is a location where water tends to collect, and it can be missed during even the most thorough of preparations for the lot.