Mise en place - Thaw and squeeze out as much moisture from the spinach as possible. Give it a rough chop. Drain the water chestnuts and give them a rough chop as well. Trim the scallions and mince them, white and green parts.
Combine - To a medium bowl, add the sour cream, mayonnaise, onion powder, garlic powder, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Mix well to combine. Fold in the spinach, water chestnuts, and scallions.
Refrigerate - To allow the flavors to develop, place dip in a lidded container and refrigerate for at least one hour, or up to 24 hours.