Set a 6-quart Instant Pot to Sauté and add the olive oil.
Add the ground beef, Italian seasoning, garlic powder, salt, and black pepper to the Instant Pot.
Sauté and crumble the beef until no longer pink, about 5–7 minutes, then drain any excess grease and return the meat to the pot.
Break the spaghetti noodles in half and layer them in a crisscross pattern over the cooked beef so they are less likely to stick together.
Pour the marinara sauce and water evenly over the noodles, pressing them down so they are submerged.
Close and secure the lid, set the valve to sealing, and cook on Manual/High pressure for 5–7 minutes.
When the cook time finishes, perform a quick release, wait for the float valve to drop, then open the lid.
Stir the spaghetti gently to combine sauce and meat, then serve warm.