For syrup, in a small saucepan, combine water, sugar, serrano peppers, and lime juice. Bring to boiling; reduce heat. Simmer, uncovered, for 3 minutes. Remove from heat; cool to room temperature.
Strain syrup into a large serving bowl; discard peppers and seeds. Stir in lime peel. Add melon balls, apple or pear, and plums; stir gently to coat. Cover and chill 1 to 4 hours before serving. Makes 6 servings.
For a milder flavor, serrano peppers may be omitted.