If I understand your question correctly, pure water is 0% alcohol, so by adding water, this will dilute your 15% alcohol solution and lower the percentage of alcohol in the solution.
Many word problems are set up like "You have some amount of 15% alcohol solution and you add some pure water. You end up with some other amount of the diluted mixture. How much 15% alcohol solution and how much pure water were mixed."
For those ones, use 0% alcohol for your pure water.