Old English tīd "point or portion of time, due time, period, season; feast-day, canonical hour," from Proto-Germanic *tīdi- "division of time" (source also of Old Saxon tid, Dutch tijd, Old High German zit, German Zeit "time"), from PIE *di-ti- "division, division of time," suffixed form of root *da- "to divide."
Meaning "rise and fall of the sea" (mid-14c.) probably is via notion of "fixed time," specifically "time of high water;" either a native evolution or from Middle Low German getide (compare Middle Dutch tijd, Dutch tij, German Gezeiten "flood tide, tide of the sea"). Old English seems to have had no specific word for this, using flod and ebba to refer to the rise and fall. Old English heahtid "high tide" meant "festival, high day."
"of, pertaining to, or cause by tides or a tide," 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; its use for "great ocean inundation caused by an earthquake, etc." is recorded by 1868. This now tends to be called a tsunami.
also riptide, 1862, "strong tidal flow in a coastal channel, etc.;" see rip (n.2). Since early 20c. it has been used mostly of strong currents flowing straight out from shore, which are not tides, and the attempt to correct it in that sense to rip current dates from 1936.