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 pot. Cook, breaking up the meat, until no pink remains, about 5–7 minutes.
Drain any excess grease, then return the cooked beef to the Instant Pot.
Break the spaghetti in half and layer the noodles in a crisscross pattern on top of the meat so they cook evenly and don't stick together.
Pour the marinara sauce and the water over the noodles, making sure the noodles are mostly submerged.
Secure the lid and set the valve to sealing. Cook on Manual/High pressure for 5–7 minutes (use 5 minutes for firmer noodles, 7 for softer).
Perform a quick release when the timer finishes, wait for the float valve to drop, then open the lid.
Stir the spaghetti to combine the sauce and meat, then serve immediately with optional parmesan and chopped basil if desired.
