I read about Nikon 16mm 2.8 and Sigma 15mm 2.8.
What you think is the best for underwater photography?Also I love half sea-half out of the sea
pictures and I would like to take this kind of pictures...
The two lenses are similar but, as I recall, the Sigma focuses closer than the 16mm 2.8. At one time, I believe it was preferred by Alex Mustard to the Nikon 16mm. But don't quote me on that!
I used to own the 16mm 2.8 and I sold it. It's the one lens I sold that I really regret selling! Also, the over/under shots I've captured were shot with the Nikon 16mm 2.8. It worked great for over/unders with my big dome lens.
You probably can't go wrong with either lens, but do remember that the Sigma will focus a little closer.
That's my two cents. Happy shooting!