Add the watermelon to a food processor or blender and puree until smooth. Transfer to a bowl and set aside.

Combine the water and sugar in a medium pot and bring to a boil until the sugar dissolves. Stir in the puree. Remove from the heat and carefully transfer the mixture to a medium bowl. Add the tarragon and stir until combined. Cover and chill in the refrigerator until cool.

Transfer the bowl to a freezer and freeze until frozen.

When you are ready to serve, let the granita slightly soften and gently mix with a fork. Scoop into individual glasses. Garnish with tarragon, if desired. Serve immediately.