Here is the full list of all 21 Ocean's Heart achievements.
Defeat Blackbeard
Defeat Blackbeard while cursed
Collect every item
Complete every sidequest
Sail to Brickenbranch Island
Learn all 4 ancient spells
Battle the Immortal Curse at the center of the world
Light the 4 Ancient Lighthouses
Fully power the Seabird's Tear
Summon and defeat the Crow Lord
Upgrade then downgrade the flail
Enter the Abyss
Defeat arborgeists of the Ancient Groves
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
Collect every monster part
Find Oakhaven's secret arena
Visit the Thorn Monk
Enter the Last Chieftan of Yarrowmouth's tomb
Collect every foraging material
Ring the Cursed Temple's bell