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