Makes 5 cups (4 to 6 servings)
8 cups cubed watermelon (seeds removed) OR 1 quart watermelon juice
1 cup freshly squeezed lime juice
1/2 cup sugar, or more to taste
Lime slices, for garnish (optional)
Mint springs, for garnish (optional)
Place half of the watermelon cubes in a blender and process until smooth.
Strain through a fine-mesh sieve set over a large bowl; discard the solids.
Repeat with the remaining watermelon cubes.
(You should end up with about 1 quart of watermelon juice).
Add the lime juice and sugar to the watermelon juice, and stir until the sugar has dissolved.
Taste, and add more sugar if necessary.
Transfer the limeade to a nonreactive pitcher.
Refrigerate until thoroughly chilled.
Serve over ice in tumblers.
Garnish with lime slices and sprigs of mint, if desired.