Set the Instant Pot to Sauté. Add 1 pound ground turkey sausage and cook, breaking it into small pieces, until browned and no longer pink.

Use a slotted spoon to transfer the cooked sausage to a plate and pour out or carefully drain any excess liquid from the inner pot; return the sausage to the pot.

Break 1 pound whole-wheat spaghetti pasta in half and add the pieces to the Instant Pot in an even layer.

Pour in the 24 ounces marinara sauce, 4 cups water, and the 15 ounces diced tomatoes (with their juices). Push the pasta down so it is completely covered by the liquids; do not stir vigorously.

Close the lid, set the steam release to Sealing, and set the Instant Pot to Manual/High Pressure for 8 minutes.

When the cook time ends, perform a quick (manual) release by moving the steam release to Venting; when the float valve drops, open the lid.

Stir the spaghetti and sauce well to combine. Let sit 1–2 minutes to allow the sauce to thicken slightly, then serve hot.