tsunami (n.) 1896, in reference to the one that struck Japan that year on June 15, from Japanese tsunami, from tsu "harbor" + nami "waves."
tidal (adj.) 1807, a hybrid formation from tide (n.) + Latin-derived suffix -al (1). A tidal wave (1819) properly is high water caused by movements of the tides; erroneous use for "tsunami, great ocean wave caused by an earthquake, etc." is recorded from 1868.