(First time visit) After docking, check out the treasure map on the ground by the scattered seaweed and to the right of the boulder. (The Boat Captain’s Key)
(First time visit) From the beach, you'll find a boarded up doorway to the left. (across the treasure map). Destroy the wooden boards to find the Mystic Gate.
Next, ignore the chain to the left for the meantime and go through the lifted gate. As soon as you enter, a Revenant and tatzelwurm will appear. After defeating them, look at the drawbridge and destroy the chain holder to lower it down. Now backtrack to the chain holding the gate and pull it down to lower the first gate and lift the other gate blocking access to the two chests.
Now jump across the bridge by the waterfall and approach the spirit that will appear there. Talk to it to start the favor “Hammerfall”. After starting the favor, operate the large gear wheel beside the spirit to open the giant gate to Veithurgard. (Refer to the dedicated walkthrough page for this favor)
After receiving the favor, turn around and look up the rock outcrop overhead, in front of the spirit's position. You should see the raven there hopping side to side.
After destroying the raven, continue forth and ignore the red vines covering the chest and the crystal by the water wheel. You can’t do anything about them yet until the water recedes further.
For now, lift the next gate to return to the open area where you fought the revenant and tatzelwurm earlier. Before looting the chests ahead, climb up the ledge to the right to find another vine-covered coffin and two corpses with a hacksilver pouch and a ship figurehead artifact (Abandon Ship set).
Now jump back down and open the two chests. The legendary chest contains a Golden Talisman of Protection (rare). You’re done at this location for now so you can head back to the mystic gate and use it to return to Brok’s shop or use the boat to continue your exploration.
This will be triggered during the Journey quest "The Sickness" and you'll be able to explore with Atreus again at the start of the main story quest "The Black Rune".