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