Place the avocado, cilantro, lime juice, mayonnaise, and salt in the bowl of a food processor and puree until smooth. If your sauce is too thick, add water, ½ tablespoon at a time, to thin it to your desired consistency. Season to taste with more salt as needed.

If you don't have a food processor, place the avocado, cilantro, lime juice, mayonnaise, and salt in a medium bowl, and use a fork or potato masher to mash until the ingredients are combined. If you prefer a thinner sauce, add water, ½ tablespoon at a time, to thin it to your desired consistency. Season to taste.