Cook manicotti as directed on package. Drain; rinse with cold water to cool.
Meanwhile, heat oven to 375°F. In medium bowl, combine ricotta cheese and cream cheese; mix well. Gently stir in onions, shrimp and crabmeat.
Spread 1/2 cup of the pasta sauce in ungreased 11x7-inch (2-quart) glass baking dish. Fill each cooked manicotti with seafood mixture; place over sauce. Spoon remaining pasta sauce over manicotti. Cover tightly with foil.
Bake at 375°F. for 25 to 30 minutes or until bubbly. Uncover baking dish; sprinkle with mozzarella cheese. Bake uncovered an additional 5 to 8 minutes or until cheese is melted. Let stand 5 minutes before serving.