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 and cook, breaking up the meat with a spoon, until no longer pink, about 5–7 minutes.
Drain any excess grease from the pot, then return the beef to the Instant Pot.
Break the spaghetti in half and layer the noodles in a crisscross pattern over the beef so they lie evenly and are less likely to stick.
Pour the marinara sauce and water over the noodles, pushing them down gently so they are submerged.
Secure the lid, close the valve, and cook on Manual/Pressure Cook on high for 5–7 minutes.
When the cook time ends, perform a quick release. Wait for the float valve to drop, then unlock and remove the lid.
Stir the spaghetti gently to combine sauce and meat, then serve.