It's great that you are questioning tinned food but I'm not sure that it's the water content that's the problem as most fresh foods are mainly water. As for the ash, I'm fairly sure that it's not actual ash added into the food! I think they just write, 'ash' on pet food instead of, 'carbohydrates' but I have no clue why!
Personally, I feed my dog a raw diet and my cat was extremely healthy, (until she lost a run in with a car) because she supplemented her diet with small, furry animals when I was too ignorant to feed her a raw diet.