Set the Instant Pot to saute mode and add the turkey sausage. Cook the sausage, breaking it into small pieces, until cooked through. Drain off any excess liquid.
Break the spaghetti in half and place in the Instant Pot. Add the marinara, water, and diced tomatoes. Make sure the pasta is completely covered in liquid.
Place the lid on the Instant Pot, making sure the vent is sealed, and set it to manual mode, high pressure, and set the timer for 8 minutes. When the time is up, manually release the pressure. Remove the lid and stir well. Serve hot.