I have read that certain small merchant vessels designated as refrigeration ships in the 1940s and 50s were practically unsinkable because of the amount of cork lining they had. One such was the famous (or infamous, depending on the way you look at it) MV Joyita which disappeared on the night of 3rd-4th October 1955 in the South Pacific, only to be found adrift over a month later on 10th November with the entire crew and passenger complement missing. Its bilge and lower decks were flooded due to a substantial leak and the boat had a heavy port list but had drifted for 4 weeks without sinking. Apparently, due to the cork lining in the hold the boat was actually unsinkable.