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