In a mixing bowl, combine the sour cream and mayonnaise.
Squeeze in the lime juice.
Add garlic powder, chili powder, and cumin.
Season with salt and pepper.
Whisk until well combined.
Taste and adjust seasoning if necessary.
Stir in chopped cilantro.
Cover and refrigerate for at least 30 minutes.
Serve with fish tacos.
Store leftovers in an airtight container.