Clean the lemons thoroughly and carefully pare off the zest, avoiding the bitter white pith. Set the peeled lemons aside. In a heat-safe jug, combine the lemon zest with the lemon balm leaves and sugar. Pour the boiling water over the mixture, stirring vigorously and pressing the leaves slightly to extract their essential oils. Allow this infusion to steep for 15 minutes. Meanwhile, halve the reserved lemons and extract the juice, straining it into a large serving pitcher. Add a few sprigs of fresh lemon balm to the lemon juice, then incorporate the cooled, strained herbal syrup. Pour in the water, or use a combination of water and ice for a chilled result. Refrigerate until ready to serve.