Well, this is looking to be a never ending story.
The power was cut off in the whole street for a few hours..!!!
So I finally decided to re-do the surfaces and thus increase render efficiency.
So one of the reflective layers is gone, but the other covers the first, so it was overkill anyway. I also masked the reflective layer with the same painted shader I used on the water, so the reflectiveness is only applied to the 'dry' seabed.
That means there is no reflection of the sun from the seabed, so no fancy water colours.
Instead, I found out that in those days there were seals in the Red Sea. So I bought a sea lion model for poser (for only a few bucks), and exported a few poses. Those will increase depth perception and realism.
I have started rendering the new setup 45 hours ago, and the crop that covers the entire affected area (about a third of the image, in the middle) is already 50% finished. So finally things are looking up!