lol. well put.

Porpoises have a fluke, the "Sea Creature" has a tail fin.
Tail fins point vertically and move horizontally while flukes point horizontally but move vertically.
Flukes belong to marine mammals, such as porpoises, dolphins and whales.

The creature also notably lacks a dorsal fin, which would make it unique among most fish, as most sea-fish,
especially large and carnivorous ones, has a dorsal fin to increase speed and agility in the water.

It appears to have skin instead of scales.
This creature could very well be a mutant hybrid between a dugong and a shark, this could be the reason for the missing dorsal fin.
Note that military scientists in the Fallout Universe did manage to hybridize creatures of a different class (Mammals and Reptiles) so to assume that they were able to create a hybrid between mammals and fish is within the realm of possibility within the physicality of their universe.

http://m dot Imgur dot com/ gallery/ p7vdw

It's spaced out because I can't post links. Still getting used to forums. And first time posting on Imgur. So it'll be choppy. Got the pictures while playing on Xbox One.