Recently I purchased a video tutorial for a sea beastie by Fugazi1968 and although it is for Silo I own I try to follow the modelling parts in Blender.
I didn’t had much spare time but actually I’m at the 8th video and can show you the results of the last 7 videos:

Let’s start with circles:

Sea Beastie 1

Sea Beastie 2

Sea Beastie 3

Make a mesh out of all these circles:
I couldn’t find something like Silo’s “fill holes” so I had to fill every single face separately.

Sea Beastie 4

Joining fins:

Sea Beastie 5

Joining tail:

Sea Beastie 6

Closing caps:

Sea Beastie 7

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