My best idea for the Many-finned Sea Serpent is that it may be a sturgeon, I was looking at a picture of a sturgeon and found that they have paired finlike scales on their sides, and a ridge on their back, just like that particular serpent. Just make the back flatter.
Awe such a cute face, but maybe not a good idea to pat it...
I think sturgeon have no teeth, so it would only gum you to death
I like sturgeons, and this guy. Their mouths freak me out a little though.