Heat oven to 375°F. Lightly grease a 9x13-inch baking dish.
Warm olive oil in a large skillet over medium heat. Soften onion for 3–4 minutes.
Add sausage. Cook, breaking it up, until browned. Drain excess fat.
Stir in garlic. Cook 30 seconds until fragrant.
Pour in marinara and water. Season with salt and pepper. Simmer 5 minutes.
Combine ricotta, 1 cup mozzarella, half the Parmesan, and Italian seasoning in a bowl.
Spread 1 cup meat sauce in the dish. Arrange half the frozen ravioli over it.
Dollop half the ricotta mixture on ravioli. Spoon more sauce to cover.
Repeat layers with remaining ravioli and ricotta. Top with remaining sauce, mozzarella, and Parmesan.
Cover with foil. Bake 25 minutes. Uncover and bake 12–15 minutes until bubbly and browned.
Rest 10 minutes before slicing. Garnish with parsley. Serve warm.