Mix together the vinegar, water, sugar (adjusting as desired) and poppy seeds in a bowl and season with salt and pepper to taste. Stir in the chopped scallions.
Place the watermelon and cucumber in a serving bowl and pour the marinade over the top. Serve immediately or cover and refrigerate a couple of hours before serving. Toss gently before serving.
© National Watermelon Promotion Board, used by permission.