Preheat oven to 325 degrees. Be sure the rack is in the middle of the oven.
Brush a 10-inch springform pan with some of the melted butter. Stir in the rest of the butter with the ground gingersnaps and sugar and combine. Press the crumb mixture into the springform pan. Bake until golden, about 10 minutes. Cook on a rack and then wrap the pan in tin foil and place in roasting pan.
Bring water to boil. Meanwhile, beat the cream cheese with a mixer until smooth. Add 2 1/2 cups sugar and beat until light. You may need to scrape down the sides of the bowl. Beat in the sour cream, then add the pumpkin, eggs, vanilla, 1 teaspoon salt, and all the spices. Beat until just combined. Pour this mixture into the crust.
Place the roasting pan in the oven. Carfully pour the boiling water into the pan until it comes halfway up the side of the springform pan. Bake until the outside of the cheesecake is set but the center is still loose. This takes about 1 hour 45 minutes. Turn off the oven and open the door for a few minutes to let out some of the heat. Shut the door and let the cheesecake finish cooking in the oven for 1 hour. Then carefully remove from the roasting pan and let cool on a rack. Run a knife around the edges, cover and refrigerate at least 8 hours, or overnight.
Bring the cheesecake to room temperature 30 minutes prior to serving. Unlock and remove the springform ring. To finish serve with a dollop of whipped cream.