Combine water and sugar in a medium saucepan; bring to a boil, stirring until sugar dissolves. Remove from heat, and stir in lemon zest and 1/2 cup mint leaves. Cover and let stand until cooled to room temperature. Pour mixture through a fine wire-mesh strainer, pressing on solids; discard solids. (Syrup makes about 2/3 cup.)
Combine watermelon, honeydew, and cantaloupe in a medium bowl. Drizzle with syrup, stirring to coat. Garnish, if desired.