but i think is a good example of fire, right?

It's okay, but it's very slow for a cannon's shot.

Most tutorials for shading just end up telling me to imagine where the shadow and shading goes. It really doesn't help at all.

lol, here's a secret for you: the reason all the tutorials are saying the same thing is because that's really all there is to drawing. There's no magic trick that's going to make you understand how to imagine forms and shadows in your mind and then put them down on paper. Follow the tutorial's advice for five or ten years, and then you'll see results, hahah!

Everyone in here gave you good advice already, too. The problem you're having now is that you're not yet skilled enough to pull things out of thin air, but you're still trying to do it. Grab a reference, and draw what you see. Don't draw what you think you see - beginner's make the mistake of taking a reference of a tree, thinking "okay, I got my reference. Leaves are green, trunk is brown," and then picking bright green and brown MS-paint colors and putting them down, which looks horrible. If you study your hypothetical tree reference, you'll notice that the trunk ISN'T brown in the picture - it might be gray, blue, green, or any other color depending on the lighting and what kind of moss and such is growing on it. Same with the leaves. Annnnnd, I just scrolled up to look at your rock and notice you actually have the classic brown and green tree too. Either I'm psychic, or I understand beginners, hahah. Take my advice, look at a real photograph of a tree. Then, completely ignore that it's a tree. Pretend the picture you're looking at is some ridiculous splatter of random colors bullshit. Then try copying those random colors and shapes, without thinking about the fact that you're drawing a tree. Beginners get tripped up because they don't actually draw what their reference is teaching them. Use the color picker if you're having a hard time seeing what the true colors in the drawing are, even. Just copy your references exactly, and try to think about why those colors and shapes are appearing.

There's no trick to drawing, but with hard practice and lots of time, you'll get better. Don't slack off when you practice, either. Half an hour of REAL practice is worth five hours of half-assedly screwing around. If you have a headache and you want to cry when you're done studying for half an hour, you're probably on the right track. Good luck. X'D

Dunno, I'm still not convinced, hahah. I myself am terrible at anatomy, so I can't really help you, beyond saying something feels off to me. =(

I like the look of this project. Your shark and barracuda weapons made me smile. I disagree that the shark's face needs changing. I love the look of bored contempt you gave him. Looks like he's thinking "wtf is this shit? = \" Much more fitting for a shark that's been plucked out of the water and carried around than a generic "evil" face, I think.