Canon 16-35 mm Mark II or Mark III?

I know terra firma photographers sing the praises of the improvement of Canon 16-35 mm EF 1:2.8L USM Mark III over its predecessor, the Mark II (Which in and of itself was a great lens).
 

But my question, given the substantial difference in cost, is there a substantial difference in quality when it comes to underwater photography?
 

Should I save my money and go for the Mark II? Or would I regret not doling out this much extra on the Mark III?

Or go with the 16-35mm f4 it's sharper than the II and just short of the III and cheaper than both.  When you put it behind a dome port and stop down to f11-16 a lot of that extra sharpness disappears.  Compared to the f4 lens unless you need f2.8 on land it provides no advantage underwater.

On 5/24/2020 at 2:54 AM, ChrisRoss said:

Or go with the 16-35mm f4 it's sharper than the II and just short of the III and cheaper than both.  When you put it behind a dome port and stop down to f11-16 a lot of that extra sharpness disappears.  Compared to the f4 lens unless you need f2.8 on land it provides no advantage underwater.

 

 

 So currently I have a canon 17-40, you think I should just stick with it, save the cash and go diving instead?

There's going to be a improvement in sharpness between the 17-40 and 16-35 f4  but whether it's enough to lay down some cash compared to going diving only you can answer.  I would think you would see some improvement, probably mainly in the corners.   If you are into that sort of thing you can find lens sharpness tests online like here:

https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=949&Camera=979&Sample=0&FLI=0&API=3&LensComp=100&CameraComp=979&SampleComp=0&FLIComp=0&APIComp=3

On this site you mouse over to swap between the two lenses,

or here:

https://www.opticallimits.com/canon_eos_ff

 On this site you need to compare tests done on the same camera - numerical results are quite different on different cameras.  You can that at f5.6 - 8 the new lens is much better , but both are quite close by f11 where you would probably shoot on full frame.

The other but is what dome do you have - to get the best in the corners of a 16/17mm lens (assuming full frame) you need a big dome, probably a 230mm dome.  In the centre the lenses seem to be quite close.

I am on full frame (canon 5D Mark IV), with an 8 inch dome port (I think that is ~ 200 mm). And looking at ebay, I can sell the 17-40 for $350-400, and get the 16-35 used for f/4 for $700-750. Interesting thing, 16-35 f/2.8 Mark II is about the same price ($700-$800). I will probably sit on this for a bit, I have no immediate need, and there is no shockingly wide difference in image quality that warrants doling out the money yet. Thanks @ChrisRoss