[trux]

Originally Posted by wet

Yes, it spirals around seasonally, I guess because of the rivers adding water and the Bosporus subtracting water. I had thought that the glacial melting from the last Ice Ages made the sea's upper water column pure freshwater, but I guess it's more like mixed brackish water now.

There is a well grounded theory that the Black Sea was a brackish water inland lake, and was catastrophically flooded from the Mediterranean Sea through Bosporus and Dardanelles some 7000 years ago. Robert Ballard (the famous oceanographer who found Titanic) found shorelines and rest of a building some 100m below the current surface. When I heard about it around 2002, I expected it to become a sensational news filling all newspapers, and causing the rewriting of school books, but strangely only couple of specialized magazines posted a few lines or an article about it. Although the discovery in fact confirms the biblical Great Deluge, it looks like it is not something jewish, muslim, and christian religious leaders and fundamentalists wanted to hear, because to them such Deluge was not global enough

There is a quote about the research here:

In a series of expeditions, a team of marine archaeologists led by Robert Ballard identified what appeared to be ancient shorelines, freshwater snail shells, drowned river valleys, tool-worked timbers, and man-made structures in roughly 300 feet (100 m) of water off the Black Sea coast of modern Turkey. Radiocarbon dating of freshwater mollusc remains indicated an age of about 7,000 years.

Read more about Robert Ballard's research for example here: National Geographic: Noah’s Flood/Black Sea Expedition--Flash
More on the Deluge history here: Black Sea deluge theory - Wikipedia, the free encyclopedia

Back to the salinity: the surface salinity of the Black Sea is at 18.5 ppt. For comparison the average ocean surface salinity is 34.7 ppt, the Mediterranean Sea 38 ppt, Red Sea 40 ppt, and the Baltic Sea (the sea between Scandinavia and the Eurpean mainland) just 8 ppt (that's already considered brackish).

I really loved diving in the Black Sea - besides its fauna and flora different than in the Mediterranean Sea, also the salinity is very pleasant - you can dive without a mask and feel no irritation in eyes and sinus like in most other seas, and even less than in freshwater. And the visibility was not that bad at all. I've been at the Black Sea about ten times (mostly end of August to late September), and the visibility of 10m was not that exceptional.

Originally Posted by wet

Is that due to silt-mud in the water or due to algae?

Some theories tell it is the algae, others speak about high concentration of hydrogen sulfide that causes the black color. It may be a combination of both. Personally I never found the Black Sea really black - until I once dove to 20m - suddenly I was in a total darkness

Originally Posted by wet

Yes. So there are many rough waves from the winds of the surrounding mountains, I guess, but no daily tide.

No tide at all, but yes, the waves may get scary and kill the visibility fast.