My last recollection of a reported "monster wave" was the one which hit
the liner Michaelangelo (or it's sister), and stove in a good portion of
the forward side of the house.
From reports, it's not necessarily the size of the wave (30m is not
common but not unheard of in open ocean storms) but the fact that it may
appear during an otherwise calm condition, unexpectedly when the ship is
least apt to be at a speed and/or heading to deal with it.

otn