Turn Instant Pot to saute setting, and wait for the display to turn to HOT
Add olive oil and onions, and cook until slightly soft
Add ground beef, break up with spatula, and cook until no longer pink, stirring frequently to make sure it doesn't stick to pot
Season with sea salt, pepper, Italian seasoning and garlic powder, and hit CANCEl
Add shells over ground beef, then add the marinara sauce and water. Do not stir, but press lightly with spoon to ensure pasta is completely covered with sauce and water
Place lid on Instant Pot, close, and make sure vent is set to SEALING position
Hit PRESSURE COOK for 5 minutes, or half the cooking time on the box of pasta
When pressure cooking is done, do a quick release, then remove lid and hit CANCEl to turn cooker off
Stir in heavy cream and Parmesan cheese, and serve