If you could only buy one lens, which one?

[MaxBottomtime]

I use a 60 alone and also with a 1.4 teleconverter for macro with a D700 and a D3x. I also use a 105mm with a 5T closeup filter. It allows me to focus the 105 within six inches of the subject.
The 60 is great for macro with an FX camera as it allows me to shoot a subject that can be as close as the glass on the port. The 105 has a sharper image but can't get as close. For really tiny subjects I may use a +5 or +10 wet diopter but I rarely do that.

60mm with 1.4 teleconverter

105mm with 5T filter

Six feet tall anchor, Sigma 15mm

Nikon 16-35mm

 

[JackConnick]

Why not the 14mm F1.8 or the 20mm F1.4 ?

Those are pretty new lenses and aren't tested or supported. They might work fine, but at $1600 a copy, I prefer the 15mm FE, which will have noticeably better corners over a rectilinear lens. The other obvious lens is the new Nikon 8-15mm FE, but from what I've heard it's only good at 8 (round crop) or 15mm. Not sure you're really gaining anything, however on the new D850 the resolution of the Sigma 15 might come into question.
BTW the Sigma 15mm FE is actually the old Canon 15m FE, Sigma bought the design from them; probably why it is so good...

 

[JackConnick]

On the 60mm, I've just never felt it was powerful enough on FX. But I love mine on DX.

 

[MaxBottomtime]

One of the things I love about the 60mm with an FX camera is that I can crop without losing pixels. The majority of my photos from 2012 through 2016 were with a 60mm.

 

[wcs563]

You made your choice already, but I concur. I use a Sigma 15mm fisheye and have the Nikkor 16-35mm. I shoot with the D800 and now the new D850. Both are excellent lenses, perfect for UW capture.

 

[PatW]

The 60 mm macro is a good lens. It focuses very quickly and it is very sharp. I shoot FX. It is not for true macro. It can do it but you have to get so close that it is impractical for that use. I have a wet lens that I can pop over the port that works pretty well. It is good for medium sized subjects like spotted trunkfish, peacock flounder, coneys, and so on.

 

[MaxBottomtime]

I also have the old Nikon 60mm lens. You can add a 1.4 teleconverter to it, allowing you to focus on subjects that touch the port. You can add a diopter to the end of the port for even smaller subjects.

Vorticeros praedatorium with a 60mm lens and 1.4 teleconverter.

Same flatworm in front of my finger in a dry glove.

 

[Imla]

Personally, I prefer the Nikon 16mm 2,8 for Open water WA. The 14mm 2,8 for caves and the 105mm 2,8 for macro.
(I use both D700 and D800)

For both 14 and 16mm you'll need to be at above f5 to make sure corners are sharp. (Goes with most WA)