I love the LX100. I sold my NEX-5R and housing and got this instead. I love the camera's ergonomics (it's very old school), and I love the assurance of the vacuum pump option on the housing!
I used the dome port most of the time, but towards the end of the cruise, I switched to the default flat port. The dome port give a wider field of view, but I found a little quirk -- the autofocus does not work well on the dome port, except when you are shooting wide (24-26mm) -- fortunately, I usually am shooting close focus/wide angle, so it is not a problem, just a "quirk". I conferred with one my very expert photography friends on the dive cruise, and we decided that it must be because the "virtual image" created by the mini-dome port is pretty close to the camera, and is closer than the minimum focusing distance for the lens, except when the lens is at a short/wide focal length (the LX100's lens can focus on something within 1.5 inches, when the lens is at 24mm). So, in theory, you can zoom in using the dome port, but then you have to manually focus, which can be difficult when you are drifting along. When I switched to the flat port, this quirk just went away -- the autofocus performed well at all focal lengths, because the camera is focusing on the actual object, as opposed to a virtual image. Sometimes, for fish that were extra skittish, it was nice be able to zoom in a bit to get the image, although that is less practical underwater than on land because of the limitations of lighting.
The colors are great, the lens is really fast/bright (F1.7 - 2.8) which helps offset the limitations on the ISO performance -- I tried to keep the ISO at 400 or below for most shots to keep the blue background from being too grainy. The resolution is great -- there is plenty of details in the shadows, which can be recovered. I did a bit of tweaking with the video clips (final cut pro), usually just boosting the color on midtones, and reducing the exposure on highlights, and that really seemed like enough to make things pop! The custom white balance is excellent and easy to use.
I have the CMC, but truthfully, I never did much macro shooting. I still have to learn how to use a wet lens correctly. One step at a time -- this camera is a great tool to learn on!!