Watermelon Agua Fresca
8 cups cubed seeded watermelon (about 6 pounds with the rind), divided
1 cup water, divided
1/3 cups sugar, divided
1/4 cups fresh lime juice
1 club soda, or seltzer water, well chilled
Lime slices, for garnish
Combine half the watermelon, half the water and half the sugar in a blender; puree. Pour through a coarse strainer into a large container. Repeat with the remaining watermelon, water and sugar. Stir in lime juice. Refrigerate until well chilled, about 4 hours.
To serve, stir in club soda (or seltzer) and garnish with lime.