For free diving during my film shooting days I would use either a Nikonos body with a UW Nikkor 15mm lens or a housed Nikon SLR with 15mm, 18mm, and 20mm lenses. All rectilinear.
You had to be cautious when entering the water from any great hight while using the Nikonos as it was easy to dislodge the lens if you gave it much of an impact, thus flooding the system.
With my current rig, Nikon D800E’s in Nauticam housings, I’ve used the same lenses as I did with the SLRs, and have taken quite a liking to the Nikon 16-35mm f/4 zoom lens and it is by far the main workhorse amongst my stable of wide angle rectilinear lenses. I simply do not care as much for the images that I’ve made while swimming with whales, dolphins, crocodiles and the other large animals using fisheye lenses.
My metal housings have all been much more robust than the Nikonos cameras, and have proven to be secure enough to make entries under more adverse conditions without fear of flooding. That said, I still prefer to get the camera immersed as gently as conditions allow.