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