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.