For me the whole discussion around pixel size, dynamic range, ISO performance, noise, etcetera isn't that important. I'm really not that good a underwater photographer and with one dive trip per year I am unlikely to reach the stage where image quality is limited by the equipment. But there are very practical issues that do matter like lens selection, housing selection, quality of view finder, strobe sync speed, autofocus performance, ergonomics, size/weight, and for many of us cost.
Where sensor size does matter to me is that it is proportional to focal length needed to get the same angle of view. So for m43 you use about half the focal length as on FF. For most applications I consider that a m43 advantage because shorter focal length means less bulky lenses and they tend to have a closer minimum focus distance, which is important underwater. However, the argument changes for macro lenses where you can already get very close and unless you dive in low visibility, getting a bit more working distance for lighting and not spooking critters becomes a benefit. It also matters for (wet) diopter users where the extra magnification depends on the focal length of the diopter AND the focal length of the lens. With the same diopter you get more magnification for longer lenses, so larger sensors have the advantage. [note some of the newer "super macro converters" may not be true diopters but I think they still work better on longer lenses]. Finally, if you aim for 1:1 magnification the maximum subject size that you can still fit in your frame is proportional to the sensor size. So with FF you can record twice the size subjects compared to m43. In other words, rather than saying m43 has 2x higher apparent magnification than FF with same focal length and distance, you can say you are seeing 2x less.
At the moment I am shooting m43 with Olympus EM5 and am very happy. I would only consider going FF if I get to do a lot more diving (or win the lottery). However, of all the arguments for and against you can make the "it has to be Canon" seems to be the weakest unless you have a large lens collection or have specific Canon needs for topside shooting. Not saying Canon is not good, I used a Canon DSLR for 6 years or so, but other brands have been progressing much faster.