Well, a wave was observed but mostly by those who didn't survive to tell of it, or a train of them, monster tsunamis, experienced on the shores of Middle-Earth, which devastated the coast. But as you say, the cause was not "a wave" but the sudden plunging of a quite large island (close to the area-size of Sumatra, almost twice that of Great Britain? *) beneath the ocean. In the Primary World, it would rank the 7th largest by land area.
JRRT stated several times his inspirations for the Downfall prominently included dreams he had repeatedly when young, of a great wave swallowing the land.
* Estimates of the actual land area size of the Isle of Elenna are based on (1) the info in "Description of the Island of Númenor" in
Unfinished Tales, (2) Karen W Fonstad's statement it was roughly 40 times the size of Hawaii's Big Island. Info on actual Primary World island surface areas for comparison was consulted in the Wikipedia entry
"List of islands by area".