Preheat oven to 350°. Lightly grease a 9x5 loaf pan. In a small bowl whisk together guava jam and water; set aside. In a bowl whisk together the flour, salt, and baking soda; set aside. Beat together the sugar and butter until light and fluffy, about 5 minutes. Add the eggs, one at a time, beating well after each addition, scraping sides down as needed. Mix in the vanilla, reduce mixer speed to low and add the flour mixture in 3 portions, alternating with sour cream, beginning and ending with the flour. Mix just until combined after each addition. Spoon in half the batter into the prepared pan, dot the batter with half the jam mixture, then top with remaining batter and dot with remaining jam mixture. Using knife swirl the jam mixture into the batter. Bake 50 to 60 minutes, or until a pick inserted into the center comes out clean. Allow the cake to cool in the pan for 10 minutes. Then transfer to a wire rack to cool completely.