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
Upgrade then downgrade the flail
Defeat Blackbeard while cursed
Defeat Blackbeard
Summon and defeat the Crow Lord
Light the 4 Ancient Lighthouses
Fully power the Seabird's Tear
Travel from Oakhaven to Goatshead Harbor in under XX:XX time without sailing
Collect every monster part
Defeat arborgeists of the Ancient Groves
Enter the Last Chieftan of Yarrowmouth's tomb
Travel from Marblecliff Monastery to the Cursed Temple without taking any damage or sailing
Find Oakhaven's secret arena
Visit the Thorn Monk
Collect every foraging material
Sail to Brickenbranch Island
Ring the Cursed Temple's bell