Here is the full list of all 15 Escape From Tethys achievements.
Defeat the IRI Assassin for the last time.
Finish the game on Easy Mode.
Finish the game on Normal Mode.
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.
Finish the game in Normal Mode in under 45 minutes.
Find all the ammo upgrades.
Find all the health upgrades.
Find every upgrade.
Explore every area on Tethys.
Unlock the high-security gate.