Mix Alfredo sauce and water in a bowl; spread about 1/3 sauce mixture into a 9x13-inch baking dish.
Season chicken tenders with garlic powder and stuff chicken into manicotti shells; arrange manicotti in prepared baking dish. Pour remaining 2/3 sauce over manicotti and sprinkle with steak seasoning. Cover baking dish tightly with aluminum foil.
Bake in preheated oven until pasta is tender, about 1 hour. Remove foil and sprinkle Parmesan cheese over manicotti. Continue to bake until cheese is melted, about 5 minutes more.