Here is the full list of all 15 Escape From Tethys achievements.
Find all the health upgrades.
Finish the game on Normal Mode.
Finish the game in Normal Mode in under 45 minutes.
Finish the game on Easy Mode.
Defeat the IRI Assassin for the last time.
Explore every area on Tethys.
Find all the ammo upgrades.
Find every upgrade.
Unlock the high-security gate.
Destroy the missile frigate.
Defeat the IRI Assassin for the second time.
Destroy the reef guardians.
Put the rogue scientist to rest.
Defeat the rock creature in the caverns.
Encounter the IRI Assassin.