The Doctor: Unsinkable?
Jabe: If you like, the nautical metaphor is appropriate.
The Doctor: You're telling me. I was onboard another ship once, they said that was unsinkable. I ended up clinging to an iceberg, it wasn't half cold.
The ship being alluded to here is The Titanic, proudly boasted to be unsinkable. The designers were so confident of this that they didn't put enough lifeboats on board to evacuate the entire ship - so as not to overclutter the deck. The ship set sail from Southampton on the 10th of April 1912 with 2200 people on board and sank in the North Atlantic 5 days later, taking 1500 people down with her.