I have a friend who is a video producer, we discuss photography a lot. When I was this picture for the first time I recalled back some of our discussions about light, filters etc. Above water to change the colour of light one uses a filter on the light source. Once the light leaves the source, its colour remains (more-or-less) the same. It’s so different underwater. The whole scene is immersed in the natural blue filter … water. The greater the distance (the ‘filter thickness’), the stronger effect the filter has.
On this picture the whole scene is lit by the ambient light. It is filtered on its way from the source (sun) to the scene and then again on its way to the camera. That’s why everything is bluish-greenish. However, there is another source of light … a strobe. The rock with the seaweed is close to the strobe and to the camera. That means the distance the light has to travel is quite short, i.e. the effect of the filter is not strong. The resulting colour of the seaweed is a mixture. It is affected by both the ambient and the artificial lights, but the artificial one, the one which is almost unfiltered, prevails.