Ascending Tides is 2 different Dungeons: Coral Aerie and Shipwright's Request.
You can access them with the finder or TPing to them in Summerset and Rivenspire.
There is no introductory quest except the ones inside the dungeon.
If you were referring to the Prologue for High Tide, I think there was another post where they pointed out it won't be released till High Tide release date is closer (I guess around May?)
i have my main house (grand topal hideaway) listed in the housing tours, it has multiple target dummies, scribing altar, and grandmaster stations (fully filled out with current game), as well as almost every antiquity furnishing on display to preview them
in progress: acquiring mundus stones (currently only have the thief)
So there is no introduction to the dungeons? I just happen open them and do them for no reason? Where’s the narrative?
There's nothing elsewhere in the game that tells us to go to the dungeons, so it's basically up to us to come up with a narrative reason why our characters were wandering around in the area and happened to come across our old friend Kaleen or whomever and agree to help out.