Beat 6 egg whites at high speed with an electric mixer until foamy. Add 1/2 cup sugar, 1 tablespoon at a time, beating until stiff peaks form and sugar dissolves (2 to 4 minutes).
Whisk together 4 egg yolks, oil, 1/2 cup water, and 1 teaspoon vanilla; set aside.
Stir together remaining 1 cup sugar, flour, baking powder, and salt in a large bowl. Fold in egg yolk mixture until blended; fold in egg white mixture. Pour batter into 2 lightly greased wax paper-lined 9-inch round cake pans.
Bake at 350° for 25 to 30 minutes or until a wooden pick inserted in the center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans, cool on wire racks.
Stir together pineapple juice, cream of coconut, and light rum; brush over cake layers.
Beat butter at medium speed with an electric mixer until creamy. Gradually beat in powdered sugar and whipping cream until smooth. Stir in remaining 1 teaspoon vanilla. Remove 1/2 cup frosting; reserve remaining frosting.
Stir pineapple into 1/2 cup frosting. Spread pineapple frosting between layers.
Stir dark rum into remaining frosting; reserve 1/2 cup rum frosting. Spread remaining rum frosting on top and sides of cake. Press coconut onto top and sides of cake.
Spoon reserved 1/2 cup rum frosting into a decorator bag with a star tip; pipe around top edge. Garnish, if desired.