Much is bounced off the surface at a low angle speeding up the light to dark sequence. I never said it was instant my wife said it was in Kenya as above. I read on land it goes from light to dark in 30 mins, I don't know how fast underwater on the reef that's why I asked.Sure, but not all of it is bounced, the the light under water still goes up or down at dawn/dusk just like out of the water. It doesn't just pop on. Dawn/dusk may be faster on the equator, but it isn't instant, it still ramps up or down