[Schöneboom]

Re the fresh water, U-boats had distilling units aboard to make water for drinking and for the batteries. Total storage capacity for potable water: 4070 liters (on a Type IXC)

About the food itself, I would factor in waste and spoilage (esp. bread and produce) and the weight of containers. Also, if the Kriegsmarine operated like other services in wartime, people might have requisitioned more food than they actually used on board, making the remainder "disappear" afterwards, for the benefit of their families. Given the increasing hardships back home, I would be surprised if this wasn't common practice later in the war.