Soda cannot dehydrate the body. However, sodas tend to contain caffeine and sugar. These substances require a great deal of fluid for the body to properly process and filter them, so soda is not the most beneficial fluid for hydration.
Sodas tend to be artificially sweetened with sugar. Sugar creates an acidic environment for the body, which can prevent enzymes from functioning properly and put strain on the kidneys. Soda is also a diuretic. Diuretics increase blood pressure and the rate of urination. This means that soda drinks pass through the body quickly, so cells do not have the time required to take up the fluid. Therefore, while soda does not dehydrate a person, it is not an efficient way of rehydrating the body.