I recently figured out how to make more or less convincing caustics in Octane and this makes it easier to do more realistic underwater scenes. Doing it the hard way (let the render engines figure it out, is very difficult and takes ages to render). Trick I used was to use a caustic map in the distribution of an emitter and use perspective mapping for projection. I added a bit of scatter volume to get some sense of depth in the scene.
Yes agree with Molly - a beautiful result. The "people" look great, and I also love the story in this one, right down to the dials on the tank showing the 02 is low. The diver really needs some help getting out of that cave - and soon! So is a mermaid really there or is her 02 so low that she is starting to dream?