This scene is lit externally with a direct light (x4) with vray area shadows plus vray environment (x9) and hdri (x2) and then supplemented with a photometric free point light (x200%) with vray area shadows in the light fitting and photometric spotlights in the pool (700%) with vray area shadows. All shadows have transparency selected.
The scene only has low settings at this stage: fixed antialiasing, GI Ir. map at standard low settings and Light Cache settings at 500/.03 with 5 inter. samples. Although I have done some higher quality samples I still need to learn about the higher quality settings for light cache in particular with these sort of scenes.
I have attempted to use Caustics, initially I think at the default settings and my machine simply ran out of memory. After fiddling with the settings with no luck I went into Vray system/object settings and removed caustics from all but the water (to generate x 5) and the ceiling planes (to receive) using settings like x200, 5, photons = 15. I was then able to render (slowly) but with little visible effect.
I would expect in a scene like this to get more dappled green light boucing onto the ceiling plus more light dancing on the base of the pool from the sunlight.