Hey Cladil!

Awesome work.
I like the water monster's almost silhouette design.

You might want to off-set the rotation of each of his segments to give him more of a serpent like movement.

Just like Soren shows in his video:

http://youtu.be/L9JuxJoac6Y

I did a quick test cause I was curious myself....
(I just offset the frames working from the end of the tail to the top.)

http://i.imgur.com/B97hc5U.gif

I made an example for you:
http://www.youtube.com/watch?v=4GZM2_UQB3Y
The more you offset the more the tail whips around. The trick is: offset keys, key at the frame you want to be the end of the animation, delete keys after that, copy all keys at end of animation to first frame.

If you want to offset the animation longer than the animation time you can just loop your animation twice before offsetting

You can offset even further to get more of a sine wave. You can also make move movement to start, before offsetting.

I'm really not sure how to get a sine wave. Anyone have some ideas?

Basically what you already showed will get a sine wave, but the values need to be pretty much spot on, which would be really hard. Only way to get a perfect Sine wave would be to be able to draw curves and constrain bones to that or use math expressions I think

Offsetting doesn't seem to get anything that looks like a sine wave. Give it a try.

you would need to translate the first bone up and down as well. But basically yeah it would give you a sine wave if offset correctly, not easy to do though