1/2 large oval watermelon
Stir the sugar, 2 tablespoons of water, lime zest, and 1 1/2 tablespoons of lime juice in a small heavy saucepan over high heat until the sugar dissolves and the syrup comes to a boil. Let the syrup cool completely, and then refrigerate it until cold.
Strain the chilled syrup into a small bowl and stir in the mint. Allow the mint to infuse into the syrup for at least 5 minutes.
Place the watermelon, cut side down, on a work surface. Using a large sharp knife, cut the watermelon horizontally to form a 2-inch-thick slab. (Reserve the remaining watermelon for another use.)
Trim away the rind and then cut the slab into 2-inch chunks. Transfer the watermelon chunks to a large serving platter, keeping the chunks in a single layer and maintaining the shape of the slab. Pour the cold syrup over the watermelon, and serve.