I’ve been experimenting with underwater volumetrics, caustics and such.
I started making this photorealistic so I can tweak the lighting and materials to give it a Finding Nemo cartoonish style
while still having somewhat photoreal textures.
The ground is really flat in my opinion
If you look at the ground of a real life sea, you’ll see waves on top of waves, that were created by water
Extreme example:
Light going trough water also gets extremely tinted (as seen on the image above)
This has a very strong effect on the ground and most of the times it gives the goldfish a blueish-orangish color instead of a white-orange one
The texture on the ground is a bit lowres and it feels very flat because it doesn’t have any bumpmaps
My last critique is, that your god rays are clearly coming from a single source just above the picture, which gives an unrealistic feeling to it, especially because you can’t see the surface of the water in the background either
nice so far…but way to go. the first render looks pretty good.
Thank you @gorbit99 for your feedback and suggestions. 
I added little bumps in the sand to give it depth. The light source previously had one spotlight, I added two more spotlights and spread them out to give the scene an even light source.
it would be nice to see the surface of the water too
This could be achived with a glass shader that has a refraction and reflection rate of water and it might give the light a blue color too
I added the ocean surface, but I might take either the sand or the surface out because the water looks too shallow for the ocean. I also added some particles in the water and turned up the lighting. Also I gave the caustics a blue sort of look by changing the right and left spotlights to a light blue, and the middle one to a light yellow.
Interesting experimentation!
I have snorkeled a lot, so I have images in my mind directly, not just from publications and documentaries.
I find the first image the most convincing. Yes it is a desolate sand bottom rather than the more “rich” photo, as commented, but I’ve seen some like yours. The aqua-color light filtering seems good, and so does the bottom from that first angle.
Your second attempt with the fish, seems fake to me and I explain: colored fish in the water, look much less colorful, with natural light from the surface the are tinted all by the blues and greens. To get the bright colors of fish in documentaries bright artificial spotlighting is cast on the objects, but that light source and it’s effects are missing. So the fish looks like a collage.
The third image goes a little further in filtering the color of the fish, in that sense its better. However the low angle to the bottom makes the bumps less convincing because they lack the texture, small relief especially in the foreground, that would show so close and at a tangent angle. About the water surface: it’s interesting how it catches daylight, but the form is closer to crinckled paper than sea-surface disturbances. There is also something un-natural when I relate the scale of it: 1. comparing the undulation of the surface to the bottom and 2. looks like it is deeper waters by the colors yet it seems really shallow when I imagine the fish size…
Hope this helps. Interesting stuff.
AFTER writing this, I read your inintial post (just viewing pics initially) to find that you are abstracting to be “cartoony” so I guess the colors must be bright and daring.
I would say have the water be only visible in the very back of the scene, like we only see a little bit of it, that would give the feeling, that the water is very deep
Anothe roption would be to simply change the angle of the shot, instead of it being paralell to the ground, make it look downwards slightly, that way you can leave the top out
According to wikipedia, Amphiprion percula, aka orange clownfishes usually live in shallow, warm waters, usually coral reefs, that usually don’t exceed 12 meters in depth, that’s still a lot, if you consider these fishes are around 10-11 centimeters long, but don’t except them to be around at the bottom of the ocean
Thank you for your feedback! It’s really helped in developing this underwater scene. 
Here I added more objects to the scene so I could pull the camera back, making the water look deeper.
Also if you notice I added a gradually density to the volumetrics to make it more murkier further back.
Next is the water surface!
How are you creating the murk falloff? Is this using World Volume or a piece of geometry? Or are you doing this through compositing?
Here’s a render with the water surface showing…
I added HDRI lighting, instead of just a solid color.
For the murk/volumetric stuff, I have a spotlight and a sphere with volumetric scatter on one layer; for the light rays I have the same thing except I have volume absorption added, and the density lowered on the volume scatter (I might decide to combine the murk and light ray volumes into one, as the murk layer is emitting light rays). To add the falloff, instead of plugging in a texture into the density and killing the GPU, I used the mist pass.