In a mixing bowl, combine ricotta, 1 cup of mozzarella, parmesan, egg, garlic, ½ teaspoon kosher salt, ¼ teaspoon black pepper and chopped parsley. Transfer mixture to gallon size baggie and snip corner to fill shells.
Spray Instant Pot insert with non stick cooking spray. Use gallon baggie to fill uncooked pasta shells with ricotta mixture, transferring them to Instant Pot as they are filled.
Add water to Instant Pot and remaining salt and pepper. Then top stuffed shells with marinara sauce. Do not stir. Set Instant Pot to cook on High Pressure for 8 minutes via Manual Mode. After cooking, use Quick Pressure Release. Top with remaining mozzarella cheese and recover with lid until cheese melts.
