Now, for some reason, shortly after I started my research on Carcharadons Astra it become a tad more mainstream (aka popular).
From my research (and a tad of my own personal input).
I'll safely assume the 1st 2 images are from running Space Sharks through Codex: Space Wolves (guy turns into shark:
MoTW. Guy riding shark: Thunderwolf Cavalry).
Now Nevelon has a few good points:
-Very assault oriented force means more chainswords.
-Beaky helmets
Now, read up on the fluff a tad (wiki and such, nothing extreme, because it doesn't exist). Most recent encounter with force in question was Badab War.
Gear was Pre-heresy and Heresy era (Mk3,4,5, ~6). Meaning beakies, studs, and iron.
You can get pre-heresy and heresy style armor (some, anyway) from Chaos
SMs, I also skeeved (yep, new word. Skeeved) some backpacks from the same set b/c they're different, and that's what I wanted.
Something else I like was that, because the
LC on Tyberos are so wicked looking (aka not normal) I looked around for alternate
LC for the assault termie squad that I would inexplicably run with him, and found these:
http://chapterhousestudios.com/index.php?route=product/product&path=85&product_id=228
and
http://www.shapeways.com/model/320409/10-28mm-power-claw-sets.html
I'm unsure of the 2nd link as to quality in any aspect, but it looks to me like they were modeled after Tyberos' personal set of teeth.
So... That's about all I've got to contribute. You can also grab some chain weapons off the Chaos set (axes?) to mix it up a tad, and look into the
FW bits.
As for the conversions that you showed in the examples above, I can't help. Fins can be done with plasticard, the shark man looks almost custom modeled (as does the shark jumping out of the guys hand). Otherwise it's all paint jobs. You can mix up your army by using bare heads, and following fluff for the bare heads (what I've found says pale skin, and all black eyes).
If you want a few other ideas, research sharks, shark toys, and oceanic culture. Should give you an idea as to what the fins look like, paint schemes, and other aspects of the topic.
That's my 2 cents, hope it helped.