Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain, return to the pot, add butter, and mash. Add celery, onions, and poultry seasoning; stir and set aside. Preheat the oven to 325 degrees F (165 degrees C). Pour stuffing mix into a large bowl and add hot water; mix until moistened. Fold stuffing mixture into potato mixture in the pot until well blended. Transfer stuffing to a baking dish. Bake in the preheated oven until top is browned, about 1 hour.