You can read about roasting fresh chiles here.
Place all ingredients except cilantro, mayonnaise and water in a blender or food processor. Blend about 10 seconds, then add cilantro little by little until blended smooth. Depending on the size of the blender/processor, it may be necessary to do in batches.
Place mayonnaise and water in a large stainless steel bowl and mix with a wire whip until smooth. Add the blended ingredients and mix thoroughly.
Place in an airtight container and refrigerate.