Here is the full list of all 10 Song of the Deep achievements.
Purchase all upgrades
Destroy the Forbidden City forever
Commune with the giant seahorse
Defeat the Fomori Boss
Activate the Deeplight
Defeat the Watcher
Meet the Merrow
Feed the Queen Leviathan
Kill a Tremorclaw Crab as Merryn
Pay your respects to the Watcher and drop a flower on her final resting place