Hi everyone. New to Blender and 3D graphics, though I do have a background in visual art and film. Am interested in learning to create photo-realistic illustrations for graphic novels and storyboards. Blender has so many unexpected procedural and un-intuitive features, am posting to see if anyone has any thoughts on where else this could be taken. Right now it’s about as simple a setup as possible - I modeled/sculpted a mesh from photos of a friend (the hard part, for me at least), placed some transparent spheres for bubbles, pointed a couple of lights through volume scattered cube and imported a background image as a plane. Am pretty happy with how the model turned out, but would love to hear ideas for how to improve the environment. Also, suggestions for fantasy artists that use Blender would be greatly appreciated.
That’s a really cool concept! I’m an illustrator who started using blender. If your goal is photo-realism, then that is a higher learning curve and a high bar! Not because it’s impossible, but I find that I spend 70% of the work on just the last home stretch of a work to get it to get out of “i can tell that was made in blender”-land and into “whoa that’s not a photograph”
For your image, there is a few things that give it away as being a 3D render and not photo realism.
The first thing I notice is the model, it is a good sculpt but it still has a small “uncanny valley” effect, probably enhanced by this being an underwater scene and the model being lit from above like that, the way the light hits the eyebrows and the nose shade (and ring) makes the shape of the nose look sharp when viewed from the smaller size (i mistrook it for low poly at first), and the eyebrows blend together with the eyeshadow. You could play with the angle of the light, make it brigher or to find ways it will effect it in a way the viewer would accept as realistic more easily. (true realism is not the same thing as what a viewer thinks is “realistic”)
the second thing is that you need to reduce the noise. I’m a noob on rendering myself so I can’t give you concrete advice, but there are more ways to do it than “just” increasing the resolution of the rendering (though I would start with that). But I can see that the noise detracts from the realism by a lot, and interfers with the shadows of the face, making the first problem worse. blender guru has some info on that in the donut tutorial during the coffe cup stage.
The third thing is the bubbles are too perfect 
check out some underwater photos with real bubbles, they are almost never really round like that, and also appear very bright, and you can make clusters with varying size, I’d probably want to do that with particles or even just a overlayed image texture. example https://live.staticflickr.com/5086/5245611577_781da01a65_b.jpg
In the donut tutorial https://youtu.be/BIUDZZMfGNI blender guru goes over using a droplet texture to fake condensation on the coffe glass, and earlier in that series he does a lot of tweaking on the liquid that might be relevant to how you do your underwater scene stuff, not sure if it will help you but it’s worth a shot.
Ok, the last but most important thing I want to say is that… you are allowed to fake things! fake as much stuff as possible if it saves you time! If you think the modelling is the hardest part… just use a photo for the face and model the hair only 
if you know you only need the one angle you can photo montage a lot and leave blender for the effects and the… blending! ok I will see myself out.