Scrub and dry the butternut squash. Pierce the skin in 5 or 6 places with a paring knife.
Pour the water into the inner pot and set the wire trivet inside.
Place the whole squash on the trivet. If it doesn't fit, cut it crosswise between the neck and the bulb.
Lock the lid and set the valve to Sealing. Cook on high pressure for 5 minutes. The pot needs 10–15 minutes to come to pressure first.
Let the pressure release naturally for 10 minutes, then move the valve to Venting to release the rest.
Test the neck with a knife. It should pierce with no resistance. If firm, reseal and cook 2 more minutes.
Remove with tongs and cool 5 minutes. Halve lengthwise, scoop out the seeds, and peel or scoop the flesh from the skin.
Serve with butter, maple syrup, salt, and pepper, or set aside for soups, purees, and pasta.