Bring the water, sugar, and salt to a boil. Stir well to combine, then remove from the heat and let cool slightly. Pour into a glass pitcher and let cool for at least 2 hours in the refrigerator.
Mix in the lemon juice, lime juice, and the mineral water. Fill two tall glasses with ice, then fill with the lemonade. Garnish with a slice of lemon and lime. You could also keep the mixture bottled for a later use, or for use as a gift.