Dome ports allow you to shoot wide angle photographs more easily. You can use them with any lens but in general you want them for wider lenses both rectilinear ones and fisheyes. The dome is acting as a big optical element and it allows you to shoot a wide scene but of a "virtual image" that is created by the dome. The virtual image is between 1 and 15 inches from the dome (depending on the dome size, larger domes lead to virtual images further away).
i can highly recommend the workshops run by Mike and Luca if you can make them, unlike many other workshops they don't take cameras with them on dives, but are there to help the students. Otherwise you can go to a Underwater Photo society if there is one near you, at LAUPS we help individuals all the time with everything from the basics to advanced stuff.
I took my 7D and Oly EM-1 on a trip to Indo. At the end, most guests thought the EM-1 photos are better. As for macro, a lot depends on what you want to shoot and how small you want to shoot. The Canon 100 and the Canon 60 are great lenses but shoot 35 mm full frame. The native OLY 60 shoots 17 mm full frame. Any diopter improves things proportionally, i.e. a 2x diopter for the Canon gets you to where the OLy is natively and the 2x on the oLY gets you to 8 mm full frame. That being said, and if you shoot raw, to me at least you can't tell the IQ apart very much. You could go Canon 5DIIIS with 50 MP and crop like mad, but I would rather carry around the smaller/cheaper system.
P.S. Going to God's Pocket in Sept, hope it is as good as the Alaska Trip.
Hope you are well and diving a lot. For me if you are shooting tiny stuff (like the small spiny lumpsuckers) then the olympus om-d e-m1 (or the new em5-II) is the best option with the only 60 macro lens. Out of the box it is 1:1 (on the smaller sensor) meaning it is 2:1 compared to full frame and 1.5:1 compared to crop sensor DSLR. Then add the CMC (or my old SMC) and you are at 4:L1 compared to full frame. I have been shooting that for a while now and for tiny stuff it is quite nice.
You can get pretty nice results from them if you use the focus stick religiously, the depth of field with them is ridiculously shallow and you need to be at just the right distance. that being said, if you nail the distance the results are quite nice.
A lot depends on your budget and how much you will dive and most importantly what you want from your pics. If you want to show your friends cool things you find, then compacts are fine; if you want to hang pictures on the wall then you will want something better, and if you want to win underwater photo competitions then you will want something different again. I would start with something like the EM-5 and the 12-50 lens (sort of all purpose) and see which way your photography wants to go. You will need the housing and a strobe or two and a focus light.
This will be nearly $2500 but will last for a long time.
My wife just finished a long (for her) documentary of some of the little stuff that lives in the ocean for a kindergarten class. We met the teacher on a trip to the Sea of Cortez and they decided to do a small film suitable for 5 year olds. It turned out great (we will send Steve a copy for the Fin Foundation) and the kids loved it; getting to the young ones early about the cool stuff in the ocean hopefully can get them to think about things in the right way.
That is why I suggested using a 77-67 mm step down ring (I have the Nauticam single ones for both the 67 and 77 ports) and the Saga dual flip adapter. You suggest that it might vignette but that seems unlikely since the lens itself is only 67 mm.
Based on my calculations, I think you will get 2.5 x magnification. That means that your 4 mm nudi will look like it is 10 mm. The big problem you are starting out with is that your camera doesn't really have macro capabilities. Compared to the CMC, you should see a tiny (2.5/2.3) effect of about a 9% increase.
The +25 will indeed work on the camera, but it will not allow him to shoot a 4 mm beasty full frame, the full frame resolution is like 60 mm wide and a + 25 will take that to at best 24 or so, not enough for the original requirement.