Combine shrimp, juice of 7 limes, and white onion in a large bowl. Cover with plastic wrap and refrigerate until shrimp is opaque, about 12 hours. Drain, discarding accumulated juices.

Blend juice of remaining 7 limes with serrano; transfer to a large bowl.

Add shrimp mixture, crabmeat, tomatoes, cucumber, red onion, jalapeños, and cilantro to the bowl. Stir in tomato and clam juice cocktail; season with salt and black pepper.