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