Combine water and sugar in a small saucepan. Bring it to a boil over medium high heat. Cook for about 30 seconds or until sugar dissolves, stirring frequently. Remove from heat, stir in the lemon and lime rind. Let sit for 20 minutes, then strain mixture in a meshed colander over a bowl and discard rinds.
Place 1/3 of the watermelon and simple syrup into a blender, process until smooth. Pour puree into a large pitcher, repeat.
When done pour in lime and lemon juice, stir and serve over ice.