Turn the Instant Pot to Sauté mode. Once hot, add the ground meat and salt. Cook, breaking up with a wooden spoon, for about 3 minutes until no longer pink.
Add the onion and garlic. Sauté for another 3-4 minutes, until softened.
Press Cancel. Add the water and use the wooden spoon to thoroughly scrape the bottom of the pot to deglaze and lift any browned bits.
Add the broken spaghetti to the pot. Pour the marinara sauce over the pasta, covering it as much as possible. DO NOT STIR.
Secure the lid, ensure the valve is set to Sealing. Select Manual/Pressure Cook on High Pressure for 9 minutes.
Once the cook time is complete, immediately perform a Quick Release by turning the valve to Venting.
When the pin drops, open the lid. Stir the spaghetti and sauce vigorously until well combined. The sauce will thicken upon standing.
Serve immediately, topped with grated Parmesan cheese.