Fill a large pot halfway with water and bring to a boil. Add the shrimp and cook until just opaque in the center, about 2 minutes; drain in a colander. Rinse the shrimp under cold running water; drain. Do not let the shrimp fully cook as the lime juice in the marinade will continue to "cook" it.
Peel and devein the shrimp; transfer to a large bowl. Stir in the tomatoes, onion, lime juice, cilantro or parsley, oil, cumin, salt and pepper. Cover and refrigerate at least 2 hours or up to 6 hours.
Place a romaine leaf on each of 4 plates. Top with onr-fourth of the shrimp mixture and the radishes. Serve with lime slices.