For Sour Cream:
Whisk all ingredients in medium bowl to blend. Season with salt. (Can be made 1 day ahead. Chill.)
Combine chili powder and paprika in large bowl. Add shrimp; toss to coat. Let stand 5 minutes. Heat oil in heavy large skillet over high heat. Add garlic and sauté until fragrant, about 1 minute. Add shrimp; sauté until opaque in center, about 5 minutes. Season with salt and pepper. Transfer to a small bowl.
Preheat oven to 350°F. Arrange taco shells on heavy large baking sheet. Bake until hot, about 8 minutes. Place shells in napkin-lined basket. Arrange half of watercress on platter. Top with shrimp. Chop remaining watercress. Place in small bowl. Place sour cream, salsa, avocados and chopped watercress in separate bowls.