Tap water is likely to contain less Ca\(^{2+}\) and Mg\(^{2+}\). The reason is because looking through all the given options, spring water, river water and sea water are natural water that has not undergone any treatment. Conversely, it is believed that before tap water is distributed for consumption, it must have been treated, thereby reducing the Ca\(^{2+}\) and Mg\(^{2+}\) content in the water.