Thaw the spinach and drain. Put in a sieve and squeeze dry to remove excess liquid.
In a large bowl, stir together the chopped spinach, mayonnaise, sour cream, Parmesan cheese, onions, water chestnuts, salt, pepper, garlic powder and dip mix.
Cover and refrigerate for at least 1 hour.
Notes
Store leftover spinach dip in an airtight conainer.