Spray a 9 1/4 by 5 1/4 by 3-inch (6-cup) metal loaf pan lightly with nonstick spray. Line the pan with plastic wrap with enough plastic wrap extending over the sides to cover the top after the pan has been filled.
In a medium bowl, whip the heavy cream, powdered sugar, and vanilla until stiff peaks form. Set aside.
In another medium bowl, whip egg whites until foamy. With the mixer on, slowly add the simple syrup. Continue beating on high speed until the mixture is glossy and doubles in volume, about 7 to 10 minutes.
Using a large rubber spatula, fold the pineapple into the egg whites. Fold the whipped cream into the egg white mixture. Gently, spoon the mixture into the loaf pan. Fold over the plastic wrap to cover and freeze for 8 hours (or up to 1 month).
To serve: Unfold the plastic wrap. Invert the semifreddo onto a cutting board. Remove the plastic wrap. Cut the semifreddo into 1-inch thick slices. Place on individual serving plates and top each slice with the crushed amaretti cookies.
In a saucepan, combine water and sugar over medium heat. Bring to a boil, reduce heat and simmer for 5 minutes, until the sugar has dissolved. Take pan off heat and cool the syrup. Any extra cooled syrup can be saved in an airtight container.