Combine the water, sugar, limes, and lemons in a large saucepan over medium-high heat. Simmer for 30 minutes, until syrupy. Remove the saucepan from the heat and cool the syrup about 30 minutes. Squeeze the juice from the lime and lemon halves into the syrup. Pour the syrup into a clean glass jar, cover, and refrigerate until needed. It will keep at least 1 month in the refrigerator.