Preheat an oven to 350 degrees F (175 degrees C). Grease 2 loaf pans.
Combine the raisins and water in a small saucepan over medium heat; bring to a simmer and cook until the raisins plump, about 4 minutes. Add the butter and cook until the butter melts into the liquid. Remove from heat and set aside to cool.
Beat the egg and sugar together in a large bowl until the sugar dissolves into the mixture. Sift the flour, baking soda, and salt together in a separate bowl; stir in batches into the egg mixture along with the cooled raisin mixture, alternating small amounts of each as you stir. Mix the almond paste and walnuts into the mixture. Pour about half of the resulting batter into each loaf pan.
Bake in the preheated oven until the top springs back when pressed, about 1 hour.