Here is the full list of all 28 20,000 Miles Under the Sea achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
You have met a toxotes
You have met a salmo trutta
You have met a plectorhinchus gaterinus
You have met a paracanthurus
You have met a boba
You have met a brycon
You have met a chaetodontidae
You have met a cyprinus carpio
You have met a balistoides conspicillum
You have met a amphiprioninae
You have met a pomacanthus imperator
You have met a falcatus
You have met a chaetodon capistratus
You have met a cyphotilapia frontosa
You have met a carassius auratus
You have met a epinephelus itajara
You have met a lepomis megalotis
You have met a coryphaena hippurus
You have met a zanclus cornutus
You have met a naso lituratus
You have met a oreochromis
You have met a herotilapia multispinosa
You have met a sebastes
You have met a lepomis microlophus
You have met a balistoides viridescens
You have met a ostracion cubicus
You have met a ocyurus chrysurus
You have met a thunnus albacares