1) Set time of day to a dusk/twilight time period, similar to the first screenshot below. Having partial cloud cover may or may not affect the results
2) Place a RHIB at Agia Marina, go into preview and drive it to a place where the sky can be seen clearly
3) Tilt the camera so that it is looking up at the sky from underwater (getting out of the boat may be necessary for this).
4) The sky from underwater should look much darker than the sky from above the water.