Here is the full list of all 15 Escape From Tethys achievements.
Explore every area on Tethys.
Find every upgrade.
Unlock the high-security gate.
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.
Put the rogue scientist to rest.
Destroy the reef guardians.
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.