In a small bowl, whisk together jalapeño, lime juice, sugar, vegetable oil and salt and pepper.
In a large bowl, combine watermelon, green pepper, onion, cucumber, cilantro, mint and salt and pepper to taste. Toss with just enough vinaigrette to lightly coat. Cover and refrigerate for at least 30 minutes or up to 2 hours.
Asian vinaigrette: Omit the jalapeño and add 1 to 2 teaspoons freshly grated ginger root; substitute rice wine vinegar for the lime juice.
Mediterranean vinaigrette: Omit the jalapeño and substitute white balsamic vinegar for the lime juice and use extra-virgin olive oil instead of the vegetable oil. Omit the cilantro and add 2 tablespoons pitted, chopped green olives and 2 tablespoons basil; reduce the mint to 1 tablespoon.
Middle Eastern vinaigrette: Omit the jalapeño and substitute lemon juice for the lime juice and use extra-virgin olive oil instead of the vegetable oil. Add 1/4 teaspoon crushed cumin, 2 tablespoons pitted kalamata olives, 1 tablespoon basil and 1/4 teaspoon chopped fresh marjoram, and reduce mint to 1 tablespoon.