What do you need help with specifically?

GOSH this looks amazing so far!!

Im not sure if I can help much, but like Fade said if theres anything in specefic I could try? :0

Love the mouth! Since you're asking for advice, though, I guess I would try to recommend that make the shading more predominant, like you did with the mouth. Looks like there should be a whole lot more grey on the body to give it more depth. Don't be afraid to use darker hues.
But that's just my opinion. Could be wrong.

Re: Gotcha. I suppose all I can say is go not be afraid with the shading. Be bold, make it dark, try different colors. If there's a specific direction the light is coming from, only let the surface areas that face it light up. For example, if your light is coming from straight above, the light focus would be on the head, back, and maybe that raised hand. The rest would be shaded to blend better with the background. That way, the important parts of the drawing really pop out.
I hope that will work for you!

re- Hmm,,, I actually really like the BG how it is! Maybe just define the seaweed shapes a bit.

As for shading, maybe make it a tad darker? The creature is underwater, so realistically, the shading should probably be a bit more green/blue-hued and would be a bit more intense/dark, as well as covering more of the body, maybe more darker the farther away it is from the "camera" (yknow, the point of view.) It'd also be cool to see some nice shafts of light shining through the murky water, making those little wave-light-like-pattern-ripple-thingies that water often makes lol. Maybe add a tad bit more texture to the skin of your creature as well.

I know for me whenever I do water drawings number one is keeping an eye on where you want your lighting from, like what Fade suggested, but for the shading I always like to use a tenth of what the background color is it helps make the creature look like it's submerged in water, because the water is all around it, don't be afraid to be bold with your shading and lighting when working with water that's actually what helps sell that the subject is submerged.

Adding particles, small bits of debris and Bubbles and other things in the water will give the picture a murky but watery look, bubbles instantly tell people that the subject is underwater.

I pretty much just repeated what everyone else said but hopefully, it helps some :)