Add the cubed and seeded watermelon to a blender and blend until completely smooth. Strain through a fine-mesh sieve to remove pulp and foam, pressing gently. Set the strained watermelon juice aside.
In a small saucepan, combine the sugar and 1 cup water. Bring to a gentle simmer, stirring just until the sugar dissolves. Remove from heat and let cool completely.
Juice the lemons until you have about 1 to 1¼ cups (240–300 ml) of fresh juice. Remove any seeds.
In a large pitcher, combine the watermelon juice, lemon juice, cooled syrup, and 4 cups ice-cold water. Add the lemon zest and fine sea salt. Stir well.
Taste and adjust only with extra syrup or lemon juice if needed — do not add plain sugar directly.
Refrigerate for 1–2 hours until fully cold.
Fill glasses with ice, pour over the watermelon lemonade, and garnish with lemon slices and watermelon wedges if desired.
