Yes, putting sand and water together in a container is classified as a mixture. This is because the physical properties of sand and water remain unchanged when added together.
A mixture occurs when the combination of two substances results in no chemical reaction and can be separated back into its original components. If the chemical structure changed, the mixture would be referred to as a reaction. Putting sand and water together is a mixture because there are various methods that can physically remove the sand from the mixture. One possible method would be to filter the mixture allowing sand to be sifted out.