I think this looks really cool, but agree it's pretty dark. It's a hard thing to deal with if you want realism. One option that would not be strictly realistic but would hint at a correct result while being brighter overall would be to lighten just the top half of the image or so, essentially dramatizing the attenuation of light at depth, and allowing you to have a brighter part of the image for mood and aesthetics. This could probably be done mostly in post work on your latest version. You'd get a brighter area where it is pseudo-realistic to have one (higher up in the water), and still the mystery and drama of the dark area at the bottom, and the contrast may enhance the overall story of the image too. Something to experiment with. Use an EXR or 16 bit TIFF as a base for post processing if you can.
- Oshyan