Preheat the oven to 350°F (175°C) and spray a 9x13-inch baking dish with cooking spray.
Season the chicken tenders evenly with the garlic salt and Italian seasoning.
Insert one seasoned chicken tender into each uncooked manicotti shell; if needed, cut tenders in half and push pieces into each end so the shells are filled.
Place the filled manicotti shells in the prepared baking dish in a single layer.
Stir the spaghetti sauce and 1/2 cup water together, then pour the sauce over the pasta so the shells are completely covered.
Cover the dish with aluminum foil and bake for 1 hour.
Remove the foil, sprinkle the shredded mozzarella over the manicotti, and bake uncovered 5 to 10 minutes more until the cheese is melted and bubbly.