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