Peel the cucumbers and cut in half crosswise. Divide each half lengthwise, then use a teaspoon to scoop out the seeds. Chop the flesh roughly. Drop into a pot of lightly salted boiling water and cook gently for 2 minutes. Drain in a sieve, then refresh under cold running water.
Halve the cantaloupes and remove the seeds. Scoop out the flesh using a tablespoon and put into a blender or food processor. Add the cucumber, lemon juice and chopped mint, then whizz together until blended. Turn out the mixture into a bowl. Pour the water into the blender or processor and whizz briefly, then stir this liquid into the melon and cucumber mixture.
Stir in the shrimp and season lightly with freshly ground black pepper. Chill for at least 1 1⁄2 hours and up to 12 hours.
Just before serving, peel, pit and dice the avocado and add the diced flesh to the soup. Snip in some mint leaves to garnish and season to taste with more pepper if needed.
Variation: For a vegetarian version of this soup, omit the shrimp and add an extra avocado.
Each serving: 231 calories, 11 g fat (2.5 g saturated), 16.5 g protein, 14 g carbohydrate, 4 g fibre