Hello Traveldave,
There are certainly a few options that are great for the Olympus E-PL10 and most of them have been mentioned below. The two main reasons we chose the AOI-UWL-09PRO were overall image sharpness and versatility.
The sharpest option would be the Olympus 8mm or Panasonic 8mm fisheye lenses and the AOI dome. This will give you a 180° field of view, but you are stuck at that FOV. I like how close you can get with a fisheye and how forgiving the focus can be. But you have to be on top of your subject. So sharks, eagle rays, etc. can be hard to shoot with this lens. Due to the nature of a fisheye lens, you can shoot at lower apertures than a rectilinear wide lens and still get sharper images. Thus you can shoot at higher shutter speeds and lower ISOs for better overall images. With fisheye, it is ƒ8 and be there. Another thing that you really can only do with a fisheye lens is split-shots or over/unders. Can't pull them off with a wet-mate lens, nor can you focus both above and below water at the same time with the Olympus 9-18mm lens.
The Olympus 9-18mm lens is a good alternative to the AOI UWL-09PRO, but you do not get the same coverage. The Olympus 9-18mm lens will get you 100° to 63°. Also, due to how dome optics work underwater, you have to shoot a high aperture to maintain corner sharpness. You want to shoot ƒ16 (ƒ11 if there is no reef/details in the corners) to get any semblance of a sharp image. This means that you will need to boost ISO or slower shutter speed for some shots. But this is a better option for skittish large subjects, like sharks.
The AOI UWL-09PRO lens will give you 130° to ~108°. So the narrowest this lens gets is where the Olympus 9-18mm lens starts. Also, since this lens has been optimized for underwater, you can shoot lower apertures and still maintain a sharper image than the 9-18mm lens. You can also take it off mid-dive and shoot tighter shots, or throw on a macro lens and get some macro in. The only downsides to this lens are cost, size, size, and weight. You have to get the float collar for it, and that just makes it bigger. The AOI-UWL-09PRO has the best non-fisheye image sharpness and versatility, but it is costly and heavy.
There are other versions of this lens, like the UWL-09 (not pro), but the image sharpness suffered significantly. We never recommend this one because of how poorly the images came out.
The AOU UWL-04A or Backscatter M52 Wide Angle Lens are another option. Certainly cheaper than the UWL-09PRO, but the overall image quality is also not as good. Since these lenses were designed for smaller sensor cameras, you do not get the same angle of coverage. You will go down to 100° at the widest and it is not as sharp as the AOI UWL-09PRO. That being said, it is sharper than the Olympus 9-18mm lens at the same apertures. With either the AOI UWL-09PRO, AOI UWL-04A, or Backscatter M52 Wide Angle Lens, you can add the AOI QRS bayonet system to them for easy removal underwater.
The Fantasea UWL-04F has been discontinued from Fantasea, so we have not tested it with the Olympus E-PL10. Knowing how they are made, it is optically identical to the AOI UWL-04A but does not have a QRS bayonet option. The current version from Fantasea is the Fantasea UWL-400F or UWL400Q. The Q includes the QRS mount, while the F includes the 52mm threaded mount.
A more inexpensive option is the Inon UWL-95 lens. This is a non-dome option and can get you 95° on the wide side. We have tested this and it does work, but there is no bayonet option for it. We have tried all the different versions of the lens and all the bayonet options vignette at full wide. A low-cost solution, sharper than most at that price, but not the sharpest and can only be threaded on. You can add a dome unit to the Inon lens, increasing the FOV to 140°. This would be the second sharpest option after the AOI UWL-09PRO lens. The downside to the Inon UWL-95 is that it is also a heavy lens by itself. A lot of glass with no air. This makes it a sharper option, but heavier in water.
When it comes to picking a lens, a lot of it comes down to what type of subjects are you looking to shoot, how close are you willing/can get to your subject and how sharp are you needing in your image? If you would like to talk directly, please feel free to give us a call. +1-831-645-1082
I hope this clarifies some of the options for you.
-Sean