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