Bring 1 cup of blueberries and water to a boil. Boil gently for 4 minutes; add butter. Stir in sugar, cornstarch, and salt. Cook slowly, stirring, until thick and clear. Remove from heat; add lemon juice. Pour mixture over the remaining 2 cups of blueberries; stir gently to combine. Pour into the baked pie shell. Serve chilled or at room temperature with a dollop of sweetened whipped cream.