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