Mix together the shrimp, mayonnaise and dill. Chill until ready to serve. To serve, place a round of the watermelon on a serving plate and top with a thin layer of the shrimp salad. Top that with another round of the watermelon and then another layer of the shrimp salad. Top that with another watermelon round. Place a cocktail shrimp on top with a sprig of dill. Repeat to create 4 Napoleons.