Preheat the oven to 350°F (175°C) and coat 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 into each end so the chicken fits inside the shell.
Arrange the filled manicotti shells in a single layer in the prepared baking dish.
Stir the spaghetti sauce and water together in a bowl, then pour the sauce over the pasta so the shells are completely covered.
Cover the dish tightly with aluminum foil and bake for 1 hour (60 minutes).
Remove the foil, sprinkle the shredded mozzarella evenly over the manicotti, and return to the oven uncovered for 5 to 10 minutes, until the cheese is melted.
