Start with the sauce, as it can sit and meld flavors while you work on the shrimp and tacos. In a food processor or blender, combine garlic, scallions, cilantro, lime juice, olive oil, sea salt, and Greek yogurt. Blend until smooth. If you prefer a thinner consistency, add cold water a tablespoon at a time until you reach your desired texture. Store it in the refrigerator until ready to serve.