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