In a medium saucepan, whisk together the water, polenta and salt. Bring to a boil over medium heat, whisking periodically. Lower the heat and let the polenta simmer, stirring constantly with a wooden spoon. Add the pumpkin and sage, and continue stirring for about 15-20 minutes, until the polenta begins to pull away from the sides of the saucepan. Stir in the parmesan cheese and butter. Serve immediately while hot and creamy, or if you prefer, place in a square dish to cool and serve in slices.