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