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