In a large pot put the shrimp, lemons cut in half, peppercorns, and a pinch of salt together. Fill with cold water just to cover the shrimp. Place the pot on high heat to boil. In the meantime put a strainer in the sink, and also fill a large bowl with ice and cold water. When the water reaches a boil the shrimp are cooked. Strain the shrimp from the water, and then plunge the hot shrimp into the ice water. This stops the cooking process immediately. Leave the shrimp in the cold water for a few minutes. Strain and reserve in the refrigerator. Peel and seed the melons, and dice into medium size pieces, or around the same size as the shrimp. Separate the diced melon from the trimmings. This is what we will make the dressing out of. In a blender combine half of the melon pieces, half of the mint, and half the olive oil. Blend until smooth. In a large mixing bowl put the shrimp, diced melon, and the dressing. Mix well and season with salt and pepper. Transfer to a serving vessel, and garnish with remaining mint leaves and drizzle with olive oil. Season the top with salt and cracked pepper.