In a nonstick frying pan, heat the water over medium heat. Add the shrimp and onion, and saute until the shrimp is opaque and the onions are translucent, 5 to 7 minutes. Transfer to a bowl, cover and refrigerate until well-chilled.
In a small bowl, add the apples and lemon juice. Toss to coat evenly and set aside.
In a large bowl, combine the celery, parsley, dill, horseradish and mayonnaise. Add the black pepper to taste. Stir in the shrimp mixture and the apples. Refrigerate until well-chilled, 45 to 60 minutes.
Just before serving, stuff the shrimp salad into the cored tomatoes. Serve immediately.