You can add some reflection in the ground caused by the water on the surface.. dont know if I say it right, cuz of my ENGLISH.
Here you can see what I mean, look at the ground https://wallpaper-s.org/17_~_Mitsubis...Underwater.htm

You can get this effect by maping a texture to a spotlight, and pointing the spotlight to the ground.

Also you should add some light coming from the top, light volumetric spotlights.. EXAMPLE: https://www.eyesontutorials.com/image...r_1024x768.jpg

And also your background should be like a gradient, a more bright blue going to a more dark blue. The bright blue towards the surface, and the dark one towards the ground.

hope this helps.