In a large skillet, stir together the water and sugar. Heat these over medium low heat, whisking occasionally until the sugar dissolves.
Whisk in the cornstarch, a little bit at a time, until smooth and any lumps stirred out. Only then, begin adding in the next bit of cornstarch. Repeat until all cornstarch has been incorporated & the mixture's smooth.
Stir in half the grapes. Raise the heat to medium high, and continue cooking until the grapes have begun to break down and the mixture's thickened.
Remove the pan from the heat, and whisk in the jello powder until evenly combined. Let the filling mixture sit until cooled.
Add the rest of the grapes to the cooled filling, stirring to evenly combine. Pour this mixture into the baked, cooled prepared pie crust.
Refrigerate the pie for 4 hours, or until set.
Using a hot knife, cut the chilled pie into slices and serve as is or topped with fresh whipped cream.