The whipped cream won't be watery when it is mixed into the mousse filling of the pie. It will keep in the refrigerator for 3-4 days. The whipped cream on top of the pie sometimes may seem a bit watery after a few days. You can add a bit of unflavored gelatin to the whipped cream which will be a stabilizer. Mix 1/2 teaspoon into 1 tablespoon cold water, stir and heat for 10 seconds. Cool a few minutes then slowly stir into the heavy cream as you are beating it.