Here is the full list of all 21 Ocean's Heart achievements.
Defeat Blackbeard while cursed
Defeat Blackbeard
Complete every sidequest
Visit the Thorn Monk
Learn all 4 ancient spells
Collect every item
Battle the Immortal Curse at the center of the world
Travel from Marblecliff Monastery to the Cursed Temple without taking any damage or sailing
Summon and defeat the Crow Lord
Light the 4 Ancient Lighthouses
Defeat arborgeists of the Ancient Groves
Upgrade then downgrade the flail
Fully power the Seabird's Tear
Travel from Oakhaven to Goatshead Harbor in under XX:XX time without sailing
Find Oakhaven's secret arena
Enter the Abyss
Enter the Last Chieftan of Yarrowmouth's tomb
Collect every foraging material
Ring the Cursed Temple's bell
Collect every monster part
Sail to Brickenbranch Island