so a 6 ft tsunami could be, on shore, 12-18 ft?
Yes, or far higher, or lower. It depends entirely on the shape and topology go the shoreline where the wave comes ashore. Even a two foot wave at sea can be terribly destructive when it hits the shore. A wave that's six feet high but five miles long... That is five miles of wave continuously coming ashore... That's a whole lot of water.