I saw, on Discovery channel I think, a show about those mystery waves that swallow ships. They think that they are generated by giant gas bubbles emerging from the ocean floor. If a ship is close enough, they can drop into the "hole" created by the gas bubble (at least a little bit), and then the resulting waves from the bubble swamp the decks and suck the boat down. Huge boats gone in like 10 seconds. They believe that these gas bubbles are the source of Bermuda Triangle disappearances, and they think they've found evidence on the sea floor of those gas eruptions.

If Anthony is right that there are 30m waves in the open ocean, then those waves are going to be at least twice as large if they reach shore with similar power.