Prepare the filling by mixing the ricotta, Parmesan, half the mozzarella, egg, spinach, herbs, salt and pepper together in a bowl. Place the filling into a piping bag (or squeeze it into the corner of a zipper sealable plastic bag with the corner cut off).
Squeeze the filling into the manicotti shells, using your fingers to push any extra filling into the ends.
Combine the chopped tomatoes and the marinara sauce in a bowl.
Pour the water into the bottom of the pressure cooker insert. Add a third of the sauce to the water. Place five of the filled manicotti shells into the sauce, making sure they are surrounded by sauce. Pour a third of the remaining sauce on top. Layer another five manicotti shells into the cooker and pour half of the remaining sauce on top. Finish by placing the remaining manicotti shells into the cooker and pour the remaining sauce over the top.
Lock the lid in place and cook on HIGH pressure for 10 minutes. Let the pressure come down naturally. Remove the lid and test for doneness with a paring knife. Scatter the remaining mozzarella cheese on top and return the lid to the cooker. Let the dish sit for ten minutes so the cheese melts and the manicotti cools enough to eat it.