Pat the shrimp dry and mince into small pieces. Place in a large bowl and toss with lime juice. Refrigerate for 2-3 hours.
Meanwhile, stir together cilantro, onion, tomato, cucumber, serranos and oregano in a medium bowl. Once the shrimp has marinaded, remove any excess moisture that has accumulated into a small bowl. Reserve in case you need to add liquid at the end.
Add the cilantro onion mixture to the large bowl of shrimp along with the tomato puree. Season with salt and pepper to taste. Serve with avocado slices and saltine crackers or tostadas.