Pour water into a saucepan and bring to a boil. Stir in couscous, cover and remove from heat. Let stand for 5 minutes, then immediately fluff with a fork (otherwise it will clump). Set aside to cool.
In a small bowl, whisk together the olive oil, cider vinegar, Dijon mustard, garlic, salt and pepper. Set aside.
In a large salad bowl, toss together the prawns, cooled couscous, red and yellow capsicums, tomatoes, parsley and feta cheese. Whisk vinaigrette to blend, then pour in about half of it over the couscous. Toss to coat and add more dressing to coat thoroughly without drenching. Refrigerate at least 2 hours before serving.