Off-soundings, tsunami are something one may experience and likely never know it. In waters the majority of cruising boats can anchor in, a tsunami can range from the un-noticed to the apocalyptic.
Most efforts to warn of and protect against tsunami and their effects have been focused in the Pacific and Indian Oceans. But mariners should be aware there are other tsunami-genic events besides earthquakes and volcanic eruptions. Several geologically significant tsunami have been the result of slip face landslides generating waves of 600-1000 feet in height.
The Boxing Day events of 2004 in Thailand, Indonesia and beyond, served to focus international attention of the phenomenon and the protection and recovery challenges involved. The following links are to information and warning sites
This image link and to the page right Great Wave at Kanagawa, an okinami (open sea or rogue wave),(ukiyo-e by Hokusai ~1830) goes directly to a site where one can sign up for warnings.