Combine the oil, one third of the jalapenos, garlic, chili flakes and a pinch of salt and pepper in a bowl. Add the shrimp and toss to coat. Cover and refrigerate for 30 minutes.
Grill the shrimp over medium heat, about 2 minutes per side. Roughly chop.
Mix together the tomatoes, cocktail sauce, onion and remaining jalapenos in a large bowl. Fold in the shrimp, crab, oysters, avocado and cilantro. Stir in the lime juice and salt and pepper to taste. Serve with the tortilla chips.