Actually, in my experience P-liners have been very weak. I've sunk a whole bunch of them over the careers, and they usually go after one torpedo.
In all my time playing, I have yet to see one. Ive sunk two battleships and a cruiser, but i have yet to see, let alone sink, a passanger liner or a flat top.
LOL... I feel your pain. I've yet to see an iceberg.