To make the dressing, whisk together all the dressing ingredients in a large bowl and season with pepper. Add the shrimp to the dressing, cover and leave to marinate in the refrigerator for about 30 minutes to 1 hour.
Halve the mango lengthwise, cutting down around each side of the pit. Cut the flesh on each half in a criss-cross fashion to make cubes, then cut the cubes away from the skin.
Remove the shrimp from the refrigerator. Add the mango, melon and tomatoes and gently stir together. Arrange the arugula and cucumber slices around the edge of a shallow serving dish, and spoon the shrimp, melon and mango salad into the centre. Garnish with mint leaves and serve.
Per serving: 264 calories, 23 g protein, 11 g total fat, 2 g saturated fat, 188 mg cholesterol, 18 g total carbohydrate, 18 g sugars, 3 g fibre, 448 mg sodium