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