Wash and dry the pumpkin Remove the top and then cut in half lengthwise. (You can remove the seeds now or wait until the squash is cooked and scoop them out).
Set a wire rack in a 6-quart or larger electric pressure cooker or Instant Pot. Add the water. Place the pumpkin cut-side up on the rack. Lock on the lid and make sure the pressure valve is set to seal. Set the cook time for 15 minutes at high pressure.
When the cook time is up, let the pressure cooker naturally release pressure for 5 minutes. Quick release any remaining pressure. Open the pressure cooker and check the texture of the pumpkin. The pumpkin flesh should be fork tender. If it's not, cook for 5 minutes more at high pressure, repeating the 5 minute natural release. Transfer the pumpkin to a cutting board until cool enough to handle.
Fit a large fine-mesh strainer over a bowl or line a regular colander with cheesecloth first. Cut the pumpkin in half. Remove any remaining seeds. Scrape the pumpkin flesh away from the skin and place in the strainer or colander. Using the back of a spoon, press on the pumpkin firmly to remove excess liquid. Set aside to drain for 1 hour.
Transfer the drained pumpkin to the bowl of a food processor fitted with the blade attachment. Blend until smooth, about 1 minute. Measure into 16-ounce portions, about 2 cups each. If any pumpkin remains, use it in smoothies.
*Pumpkin substitute: If you can't find a sugar pumpkin, use 2 acorn squash instead, 3 to 4 pounds total.
Storage: Keep the purée in the refrigerator for up to 2 days or place the fully cooled purée into a plastic freezer bag. Flatten and freeze for up to 3 months. Thaw in the refrigerator overnight before using.