I'm not sure about the U-Boats, but the US sub crews were the best fed in the navy. I believe the fleet boats (or most classes of them) had a refrigerated compartment on the lower deck. I am not sure about the location, possibly connected to the pump room under the control room (between the forward and aft battery compartments). The officers even had a pantry next to the ward room.
There was usually plenty of fresh water (when not contaminated). Even enough for limited showers and a washing machine for laundry. There was always coffee available.
I have schematics of the Gato, Balao, and Tench class boats but never seen, or looked for, a refrigerated compartment. Would take me awhile to dig them up.
Canned goods were stowed in every nook and cranny throughout the boat. Fresh fruit didn't last long though.
Usually it was the fuel situation that decided the length of the patrol.
On a side note, whenever one of my crew gets killed in action, my galley stewards would announce " Fresh meat is back on the menu boys"
