“And spare a thought for the West Indies, Bahamas, etc. which would be utterly wrecked.”
Not necessarily, db. It depends on the topography of the individual islands. Tsunamis cause damage where there are long, gradually sloping approaches from the deep water to the land. The very long wavelength of the tsunami only does damage when it meets resistance from a rising seabed. I was in the Maldives a week or so after the Boxing Day tsunami of 2006. Whilst some (of the 1,200) islands sustained damage, many of them survived relatively unscathed. The island on which we stayed had a resident marine biologist. He noticed, on the morning of the tsunami, that the tide had been “sucked out” of the island’s lagoon and beach bungalows which were usually in 8 to 10 feet of water, were high and dry. He knew what was happening. Over the next hour or two the water was replaced and the level rose so as to flood many parts of the island. But this then subsided. The process was repeated two or three times, each time a little less severe than the previous. By the late afternoon things had returned to normal and the only signs that anything unusual had happened was that debris and seaweed had been deposited across the island well above the usual high water mark. He had copious photos of the phenomenon. Nobody had been injured. A party of divers who were underwater had been “sucked down” by the enormous currents but they had the presence of mind to stay down and perform a controlled ascent (thus avoiding “the bends”).
This was the manifestation of the ultra-long wavelength the tsunami produced which, as you explained, would be almost unnoticed on the surface of the deep ocean.