In a small saucepan over medium-low heat, bring the water and sugar to a simmer, stirring occasionally, until the sugar dissolves, about 5 minutes.
Cool the simple syrup to room temperature.
Working in batches, process the watermelon in a blender until it becomes liquid. Strain it through a fine-mesh sieve. You should have 4 to 6 cups watermelon juice. Discard the solids.
In a pitcher, stir together half the simple syrup, watermelon juice, lemon juice, and salt. Refrigerate until cold.
Stir the watermelon lemonade to recombine. Taste and, if desired, add additional simple syrup. Pour into ice-filled glasses.
Notes
What You Need To Know About Making Hard Watermelon Lemonade
To give this lemonade a little adult flair, add a shot of chilled vodka to each glass before pouring in the lemonade.