Aquashifts
Iguanas in a submerged garden

The marine reptile fauna has drastically changed in Antarctica since the END: while some once common species never made a return (sirenians and turtles), other new taxa have made their first appearances, like sea snakes and even one species of crocodiles.
However, the most unique marine vertebrates are aquashifts (clade Mariguanidae), placid reptiles of the shallow seas. They are distant relatives of the mariguana, a marine lineage of iguanids that was able to survive the END, mostly thanks to their wide distribution and plastic diet, diversifying in a moltitude of shapes. 

Marbled aquashifts (Litoguana bipoda) are the most common species in Antarctica and the most herbivorous one; they've lost their hind limbs, using the long tail as propulsion and forelimbs to change direction. While not the fastest aquatic creature, marbled aquashift can reach almost 30 km/h for very short distances on certain occasions (e.g., escaping from predators).
They are particularly abundant in the Infinite Archipelago and the Fire Bridge, where vast marine prairies can be found. While marbled aquashifts rarely move in cold waters, some species of aquashift can wander near the south pole during the summer, in search of other sources of food, like kelp and fish.
They're completely viviparous, with an average gestation of ten months, losing any type of relation with the land.
It's not rare to see dead aquashifts washed ashore during late summer, full of wounds caused by intraspecific fights: males become very aggressive and territorial during the breeding season, causing an increase in mortality for young males. The winners of these fights, usually old individuals, can often reproduce with over 20 females in the entire season. This behavior curiously has lots of similarities with holocenic elephant seals.