The title reminds me of a few very funny turtles, i absolutely adored them. This one doesn't look so friendly or high .

But the capture is just absolutely perfect, how often do you see good shots of turtles? This makes the image as original as it can be. And i have to say the lighting here is just perfect as i assume you didnt have the perfect lighting conditions.

I like the depth of field because it gives some sort of a natural look to being underwater, as farther as things get away from you the blurrier they become

Choosing for grayscale was a wonderful decision, i think it really works well as it is a very clean shot, although i'm very curious how it looks in colour.

But overall i think this is a brilliant capture, very unique and very well executed!