8-15 nikkor fisheye

To my Seacam D850, I already have a small wideport dome it does not perform very well

with my 14 mm nikkor, so I want a better dome if I buy a superdome, will I then be able to

use an 8-15 mm fisheye in the superdome as well as my 14 mm, or do I have to buy 2 expensive

dome ports, the super dome and the Fisheye dome. 

No reason you can't use the big dome for both, though it's not listed in Seacam's port charts for the Nikon 8-15  so you would need to work out how much extension was needed for the fisheye lens.   Of course it  it will make some CFWA shots difficult due to the size of the dome. 

If you already have a small wide port you should be able to use it with the 8-15 lens for times when you want to do CFWA.  Nauticam specify a 140mm dome for the 8-15 fisheye.

Yes, the Superdome works very well with the 8-15mm. It takes a 20mm extension ring. It's best to have a Superdome that has removable shades. All of the newer ones do.