Wapiti

I have no experience with the 12mm prime, but I can confirm that the 12-45 works just great in a small dome with AOI PEN style housings. While I prefer the Inon dome meant for the Oly 8mm, I also tested it with the Zen dome meant for the 9-18. Both work great, but domes for the 8mm allow you to cut down on the number of ports in your kit. You will, however, need an extension ring. Since adopting the 12-45, my 12-40 and 170mm dome stay home collecting dust. Speaking of, the 170mm dome doesn't work so good with the 8-25; an 8" dome is better, but at that point you're so big and floating that you might as well go small and swim back a bit. I know, easier said than done- sometimes you just need wide rectilinear.

I have the OM1-mks I and II, and while I've used the mkI underwater I don't have a ton of reps with it yet. I can say that both do very well with subject detect topside. I've heard that subject detect works well underwater, and my recommendation would be to set it up for a medium sized box. Meaning, not the smallest single focus point, but the box that takes up about a third of the frame. This will limit unintended targets, and I would also suggest using the focus limiter to keep the camera from hunting outside of your viz limit. Prior to the OM1s, I shot the E-M1mkIII underwater and I too seem to gravitate towards fish portraits. That camera did just fine locking on to my intended target with C-AF, and based on topside experiences with all 3 bodies, I can't imagine the OM1s would do anything but improve the hit rate.

Nope, most (but not all) of the AOI housings for Olympus micro four thirds cameras take Olympus PEN ports standard, and they sell an adapter to the Olympus OMD ports (which in turn can be adapted to use older Olympus ports for their 4/3 DLSRs). For the lenses you mentioned, you could find a couple different options used if you're patient. There's a ZEN WA-100 port on eBay right now for the 9-18 (not mine, and I didn't check who the seller is), and other manufacturers of PEN dome ports include INON and Athena. If you're buying new, AOI ports are reasonably priced and readily available.

To tag on to John E's post, if you're cheap like I am the AOI housing for the OM1 is a great option because it opens you up to the whole world of PEN, OMD, and 4/3 ports- some of which have been use for over a decade now, and can often be found used at a great price.

Moving over from Wetpixel- same user name there. I also post at ScubaBoard in the Olympus Outlet forum as Furnari. I'm a long time Olympus user, currently with OM-1mkI and mkII bodies and a variety of lenses. Mostly topside, but I can occasionally get in the water here in Oregon and other PNW locations.