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