In a blender add 4 cups of the watermelon chunks with 1 cup of water. Blend on high speed until the mixture is smooth. Transfer drink to a pitcher (see note)
Note: If the watermelon has seeds, pass the juice thru fine mesh strainer to remove the blended seeds.
Repeat process with remaining watermelon.
Add the ½ cup sugar and lime juice to the pitcher. Mix well and taste for sweetness. Add more sugar according to taste.
Pour into a tall glass filled with ice and garnish with fresh watermelon (optional). This drink is best if consumed within 1 to 2 days max.