Put macaroni, water and salt in the Instant Pot. Close the Instant Pot lid and turn the valve to a Sealing position. Press the "Pressure Cook" or "Manual" button and set the time to 2 minutes on High pressure.
When it's done cooking, let the pressure release naturally (NR) for 5 minutes with the Instant Pot still plugged in, then quick release (QR) the remaining steam and open the lid.
Press the "Saute" button of the Instant Pot.
Add the heavy cream and stir. Add the Cheddar cheese and stir until cheese is melted.
Remove mac n cheese from the Instant Pot and serve immediately.