Abzutani
Abzutani is based on the Plesiosaurus, and named for the primordial Mesopotamian God of Fresh Water from underground aquifers which were thought of as being from a deep underground Ocean of Freshwater that was the source of all Freshwater on land.
Tiamatani
Tiamatani is based on the Serpentisuchops, and named for the primordial Mesopotamian Goddess of Salt Water and the Ocean who with Abzu gave birth to the first generation of Gods and then gave birth to an army of Dragons and Monsters to get revenge against the gods after they killed Abzu to take his power.
Phostovanthos
The Phostovanthos is based on the Phosphorosaurus, a small nocturnal Mosasaur with advanced sensitive eyes for chasing prey in poorly lit conditions. They are small and quick able to launch themselves up from the dark of the deeper ocean and snatch fish before they noticed they were there.
Thalathiras
The Thalathiras is based on a Liopleurodon (AN ENCHANTED LIOPLEURODON CHARLIE!) While it looks like a Mosasaur, it was actually a relative of the plesiosaurus with a shorter neck and a longer jaw. It's four flippers (Now they have Six Flippers!) gave them very good acceleration making them a successful ambush predator and studies of it's skull have shown that it had a powerful sense of smell to identify the sources of certain smells in the water.
Blue Leviathan
The Blue Leviathan is based on the Mosasaurus, a marine reptile that probably began as something similar to a Monitor Lizard, but with Dinosaurs dominating on Land they found their own niche in the ocean and became massive aquatic Predators that at full size had no threats...except one but I will get to the Kraken Later.
The Leviathan is a sea monster that appears in the Hebrew Bible that is said to be so powerful that humans have no hope of stopping it; It is the embodiment of primordial chaos, it's skin is too hard to pierce by mortal weapons, it breaths fire, and is overall indomitable by mortal beings, only God himself can destroy the Leviathan who will be the last thing killed at the end of time and served at a feast for all the righteous in heaven.
Botoatl
Botoalt is based on the Mixosaurus, a small Triassic Ichthyosaur who's name means "Mixed-Lizard" because it seemed to transition between an Eel shaped Ichthyosaur to a Dolphin shaped Ichthyosaur. In Soaring I imagine they have the power to shift between Eel and Dolphin form, one is for swimming fast in open water, the other is for swimming agilely in tight reefs and kelp forests. It gets it's name from the Boto Encantado, a mythical Amazon River Dolphin who has the power to transform into a human leading humans back to the water with them and turning them into Dolphins.
Cetus Levianthan
The Cetus Leviathan is not based on a specific species of Mosasaur but just off the idea of what if a mosasaur species filled the niche of a killer whale. The Cetus is a giant sea monster in Greek mythology that Princess Andromeda was almost sacrificed to before being saved by the hero Perseus. They also lent their name to the scientific name of Whales "Cetaceans"
Michipitsi
I actually can't remember what this one is based on, I must have forgotten what it was supposed to be and I am now having trouble matching it to any of the marine reptiles I have references for but it has more leg like flippers so I imagine it can probably waddle on beaches for a short period of time or use it's flippers to walk around the sea floor in the shallows. It's name is literally "FISH LIZARD" which I guess is also the meaning of Ichthyosaur...
Botonaue
The Botonaue is based on the Ichthyosaurus, a marine reptile that has the fins like a fish and the general body plan of a Dolphin and filled a similar ecological niche being speedy predators. The name comes from a mix of the Amazon River Dolphin and Nanaue a Hawaiian myth of a shapeshifting Shark Man. The Boto Encantado can be identified in human form by a blowhole on the top of their head so they usually wear a hat, and Nanaue can be identified by a Shark Mouth on his back that takes over whenever he enters the water.
Akamptlaimos
Akamptlaimos is based on the Tanystropheus, a protosaurian archosaur that is related to dinosaurs but is not actually a dinosaur but was related to them, and extremely recognizable by their elongated neck that could grow up to 10ft long made up of 13 individual extremely stretched rod like vertebrae, and we see long necks pop up many more times in multiple marine reptile species suggesting that is was a successful strategy to be able to catch prey in the water at a distance by moving your head and neck rather than your whole body, but unlike the successful Plesiosaur, the neck of Tanystropheus was rigid so it could not possibly move it's neck around as easily. This is a design that seemingly should not work but evolution went this way for Tanystropheus because it was a successful strategy for them for Millions of years. It was capable of moving around on legs with it's terrestrial limbs but was more comfortable floating and swimming in the water and eating fish.
The Syodrekgandr are not based on any specific prehistoric creature, it's kinda plesiosaur-like but really it's a sea serpent or sea dragon with a name inspired by Norse. Sjodreki means "Sea Dragon" and Gandr like in Jormungandr means "Elongated" which can refer to may things that are long like Snakes.