Fill a large pot ⅔ full with cold water. Add the bag(s) of crab boil and bring to a rolling boil. Turn down heat if needed to maintain a low to medium boil. Add in potatoes and sausage and allow to cook for about 20 - 25 minutes, add in corn. When potatoes have softened to fork tender, add in shrimp and crawfish (optional) and cook to taste, for about 3-5 minutes until shrimp are pink throughout and done.