There are two ways:
The least expensive is to find a Sea & Sea 16mm lens originally designed for the Nikonos 35mm lens. Make sure it has a 58mm thread. They're available on eBay for about $100. Then buy a 46-58mm step up ring for about $8. The setup works well, but you do have to attach the wide angle underwater (it needs water to focus). The Sea & Sea is extremely wide. You will need to zoom in just a bit to avoid vignetting (black corners). I'm happy with the results. I learned about this through another member, Gary from Colorado.
I've attached a couple of photos. Forgive the focus on the first, it was aimed at the Nikonos.
If you want to be really flexible, Inon makes an AD-mount adapter. Then you have a choice of of the 105AD lens or the 165 fisheye. The 105 sells for $259 while the fisheye is $369.