1) For the gelatines: Use a melon baller to portion the melon into small round balls. Divide the melon balls and half the raspberries evenly among 4 tall narrow glasses or dessert cups, about 230ml each.
2) Sprinkle the gelatin over 250ml of the Prosecco and stir to combine (fresh sparkling wine will bubble up, so do this in a medium bowl with room for froth). Let stand until the gelatin softens, about 5 minutes.
3) Toss the remaining raspberries with the sugar and water in a medium saucepan and heat over low heat until raspberries break down and sugar dissolves, about 5 minutes. Pour off about 3 tbsp of the raspberry juice, and reserve for flavoring the whipped cream.
4) Add the remaining Prosecco to the berries in the saucepan and heat over medium-low until the berries break up and flavor the wine, about 2 minutes. It's best not to let the wine boil. Stir in gelatin mixture until dissolved, about 1 more minute. Strain and press through a fine mesh strainer to remove any fruit pips. Pour Prosecco gelatin into glasses over the fruit and refrigerate until set, about 3 hours.
5) For the cream: Just before serving whip the cream and sugar until soft peaks form. Fold in the reserved raspberry juice. Dollop the cream on top of each set gelatin and serve.