Definitely get the flat port and the Sony 28mm lens since you already own the WWL-1 That 130 FOV fits very nicely between the 16-35mm zoom and fisheyes. The 16-35mm is a great zoom that both the Nikon and Canon dSLR shooters have used for years for wide angle. Sony gives you two lens choices for their 16-35’s.
One thing to note with the 28mm and the WWL-1; even with the buoyancy collar, you will have a very negative system. I have recently switched from using Sea&Sea housings with various iterations of the Canon 5D cameras to the Sony a9 and Nauticam housings. Overall, love the Nauticam system and the options to experiment with various wet optics.
So naturally, another wide angle option is the 16-35mm with extension rings and dome port. I almost went this route but then bit the bullet and went with Nauticam’s WACP. Todd Winner just recently posted an article about using the WACP with use on Sony full frame cameras on the Nauticam website. A few of my images from Socorro are used in that article. Although the WACP is large and heavy topside, I now have this rig balanced extremely well in the water and feels much better to me than when I was using my Canon 16-35, 80mm of extension and dome port or the WWL-1 with the Sony a9. Plus you get all the angle of coverage of the 16-35 and then SOME!!!!
I have also used the Nikonos 15mm with the Sony a9. That is a compact and very fun system to use. I have found that my results are better and extremely sharp when subject is very close to lens. I would still be shooting more with the Nikonos 15mm if it weren’t for the WACP. I’m still shooting and learning how to maximize my results with that setup.
As for true fisheye options, I’ve been trying to get my hands on an older Nikonos mount fisheye that was made by Sea&Sea back in the day. It was a 12mm fisheye and I just missed a posting here on the Wetpixel Classifieds. The lens went quick!!!
I don’t have underwater experience with the Canon 8-15mm fisheye and my Sony a9, but here are my thoughts and topside observations/results on the Canon 8- 15mm fisheye and the Sigma 15mm fisheye. First off, I loved shooting my older style Canon 15mm fisheye and have produced stunning results with that combo. So naturally, I looked at how I could pull this off with the Sony/Nauticam system. With Sigma recently releasing a lineup of lenses dedicated to the Sony full frame mount, I’m holding out that either Sigma or Sony releases an 8-15mm fisheye equivalent in the sometime near future. So I decided to experiment with the Sigma 15mm fisheye as it’s half the cost of the Canon 8-15mm. First, you will need an adapter to use either the Canon or Sigma fisheye. I experimented with both the Sigma MC-11 and the Metabones v5. Autofocus speed is much too slow to my liking. Topside with the Sigma 15mm, the MC-11 acquired focus somewhat faster than the Metabones but still too slow for me. So, I decided I would go manual focus. Manual focus along with using focus peaking and focus magnify was awesome and quick topside. So I then ordered the focus ring for the Sigma 15mm.
Topside testing in the housing did not go well. The focus knob that’s on the 35.5mm extension ring that you will need, was not smooth or consistent in how the teeth messed/aligned with and turned the focus ring. Since my plan was to use this with faster moving subjects, I have decided not to try and experiment with this combo underwater. Also, getting the lens and adapter off the camera during disassembly is not an easy endeavor. However, if your goal is CFWA and more stationary subjects, you may still want to experiment with the fisheyes. I will wait it out and for now, I’m excited about having that 130 FOV and learning maximizing the capabilities of the sony a9 paired with the WACP.
Hope this helps and best of luck.