Make a simple syrup by combining water and sugar in a saucepan over medium heat. Bring to boil, then reduce heat and simmer for 5 minutes until sugar has dissolved. Take pan off heat and cool.

In blender, combine mint, lemon juice, 1/4 cup simple syrup and Amaretto. Blend until smooth to make dressing. In large bowl, combine watermelon and cantaloupe. Add dressing and toss.

Refrigerate remaining simple syrup for another use.