Sprinkle the gelatin into the cold water in a small bowl and let it soften.
In a large bowl, whisk the egg yolks with the granulated sugar, salt, and vanilla. Slowly whisk in the hot scalded milk, then transfer the mixture to the top of a double boiler. Stir and cook until the custard coats the back of a spoon.
Add the softened gelatin to the hot custard and stir until fully dissolved.
Pour the custard into a large bowl and refrigerate until it becomes syrupy and thickened, about 1 to 2 hours.
Use an electric mixer to beat the egg whites until stiff. Fold the beaten egg whites and 1 cup of the whipped heavy cream into the chilled custard, then pour the cream filling into the baked pie crust and spread to the edges.
Top the pie with the remaining whipped cream (sweeten if desired) and sprinkle the shredded coconut evenly over the top.