There are a few other fisheye and wide angle options out there, however I have not tested them so cannot comment on them. Sony 16-35mm (not super wide, but is super versatile with the reach to 35mm).
The difficulty with manual focus, is that your focus point needs to be set before you close your housing. However, with the speed and accuracy of autofocus on the new Sony A7iii and A9 cameras, focus reliability is less of a concern that it was 5 years ago. Historically, fisheye is shot in manual focus, mostly to give you reliable results.
However it is the smallest lens of the options, meaning it will give you the most compact setup in a housing. The 8mm is a really nice compact option, however only covers the APSC sensor size, meaning you won’t be taking advantage of the full frame sensor size of the A7iii/A9. The black, Sony E-mount Rokinon 8mm f/2.8 UMC Fisheye II is a prime fisheye lens specifically designed for APS-C sized image sensors. However the downside to these lenses is that they are manual focus, and manual aperture only. The Rokinon/ Samyang fisheye lenses are definitely an enticing option at the low price point. This is my personal setup, mostly because I also use the lens on my RED Scarlet-W camera for video/cinema work. It fits perfectly in our M-D6.3 dome port. The autofocus is amazingingly fast, even though it is not a native Sony lens. The 8-15mm range provides opportunities to shoot circular, as well as 180 degree full frame fisheye. It is significantly more expensive than other options, however you may already have the 8-15mm if you are a recent ‘Canon to Sony’ convert, or you may be able to find a cheaper second hand one from someone also switching to mirrorless. If you want the highest quality fisheye lens, and budget isn’t of concern, then the Canon 8-15mm fisheye coupled with the Sigma MC-11 adapter is the ultimate solution. Only slightly lighter and smaller than the 28mm + converter. If your budget can’t stretch for the 8-15mm, and you don’t think you will need autofocus, then the Rokinon 12mm f2.8 is a great option.
If you are after the highest quality option, and have no budget constraints, then go for the Canon 8-15mm + Sigma MC-11 adapter. There are a handful of options out there, however in my opinion, at the moment there is no ‘perfect solution’. A removable petal-shaped lens hood is included.One of the most asked questions lately is ‘what fisheye can I use with the A7iii/A9’? I t offers a wide, easy to grip focus ring for smooth manual control and bright aperture, distance and depth of field markings on its metal barrel. This Sony FE Full Frame mount lens is manual focus lens but due to its featured AE chip provides focus confirmation in the camera's viewfinder and enables the use of Auto exposure mode and P/A/S/M modes. Its internal focus system enables its compact design and keeps the front element from extending during focus. Highly effective nano crystal anti-reflection (NCS) coating applied together with UMC coatings improve light transmission and reduce ghosting. The lens' sophisticated optical design consists of 12 lens elements arranged in 8 groups including three elements made of low dispersion ED glass and two aspherical lens elements to minimize coma and chromatic aberration.
The 12mm f/2.8 ED AS IF NCS UCM Fisheye Lens for Sony A7 Series Full Frame from Samyang provides a 180º diagonal angle of view on full-frame cameras for a ultra-wide angle, distorted perspective that is useful for architectural interiors, landscapes and creative experimentation in any photographic application. The f/2.8 maximum aperture is effective in low light situations and enables some measure of shallow depth of field control.