I've made this recipe many times, and I've found that 2 1/4 cups water is the right amount of water to add, along with one 24-ounce jar of marinara sauce. It's enough water for the pasta to cook and for the sauce to spread evenly over the noodles without diluting the sauce. And, it doesn't leave too much water still sitting the pot when the cooking time is done. It's also enough water to prevent the meatballs or sauce from burning to the bottom of the pot. I have never had a burn notice when I cook this pasta.