Cake: In a medium bowl, mix the apples and sugar. Let stand about 30 minutes.
Preheat oven to 180 degrees C. Grease and flour two 25x38cm cake tins.
Stir the water into the bowl with apples. Blend eggs and margarine in a small bowl and mix with the apples. Mix in the flour, bicarb soda, cinnamon and salt. Stir in the sultanas and walnuts. Transfer to the prepared cake tins.
Bake for 1 hour in the preheated oven, or until a toothpick inserted in the centre comes out clean. Cool slightly before turning out of tins.
Icing: In a medium bowl, blend icing sugar, hot water and vanilla essence. Drizzle over the cakes while still warm.