Squeeze the lemon halves into a large pot, and toss in the halves. Add the garlic, bay leaves, peppercorns, and salt, then fill with water to half full. Bring to a boil over high heat, then stir in the shrimp, and turn off the heat. Allow the shrimp to poach until they are no longer translucent in the center, 3 to 5 minutes. Drain in a colander in the sink, then place the shrimp on a baking sheet, and refrigerate until cold, about 30 minutes.

Stir the coconut milk, lime juice, serrano peppers, chopped cilantro, and onion together in a non-metallic bowl; season with salt to taste. Cut the chilled shrimp in half lengthwise and stir into the coconut marinade. Cover and refrigerate 30 minutes before serving. Garnish individual servings with the cilantro sprigs and lime wedges.