In a blender, combine mint, lemon juice, simple syrup, and amaretto. Blend until smooth.
In a large bowl, combine the watermelon and the cantaloupe. Add the vinaigrette and toss. Transfer to a serving bowl and serve.
In a saucepan, combine water and sugar over medium heat. Bring to a boil, then reduce heat and simmer for 5 minutes, until the sugar has dissolved. Take pan off heat and cool the syrup. Any extra cooled syrup can be saved in an airtight container in the refrigerator.
Recipe courtesy of Giada De Laurentiis