Set the Instant Pot to Sauté. Add the ground turkey sausage and cook, breaking it into small pieces, until no longer pink and cooked through.
Drain any excess liquid from the pot if there is a lot of fat or liquid remaining.
Break the spaghetti in half and arrange it in the pot over the cooked sausage.
Pour the marinara sauce, water, and the canned diced tomatoes over the pasta, ensuring the noodles are submerged in liquid.
Secure the lid and set the vent to Sealing. Cook on Manual/High pressure for 8 minutes.
When the cooking time is up, perform a quick (manual) pressure release according to the manufacturer's instructions, then remove the lid.
Stir the pasta and sauce thoroughly to combine and serve hot.