Sift powdered sugar into a medium-size, heavy, nonstick saute pan, and set over medium heat. Cook until sugar begins to dissolve, stirring occasionally, about 5 minutes. Continue cooking, stirring often, until syrup turns a deep golden brown, about 5 minutes longer.
Whisk in whipping cream and water. (Be careful–this mixture will bubble up.) Reduce heat to low. Cover and cook, stirring occasionally, until any hard pieces dissolve, about 20 minutes. Remove from heat.
In a clean, dry, medium-size bowl, beat heavy cream to soft peaks and set aside.
Place 1 scoop of vanilla ice cream into each of 6 sundae dishes. Spoon about 3 tablespoons of warm caramel sauce over the top. Spoon a few heaping tablespoons of blueberry preserves. Place a second scoop of ice cream on top. Top with more warm caramel sauce and a few heaping tablespoons of cherry preserves. Spoon whipped cream on top, followed by toasted pecans (if using).