I will start by saying that my equipment is not the limiting factor in getting better images, its the grey matter behind the viewfinder that holds them back! I see great images all the time shot on all formats.
I'm a big fan of the m4/3 system in general and Olympus gear in particular - I've had three of them. I currently use an E-M1 mk II in a Nauticam housing. I shoot stills exclusively and macro with the 60mm lens probably 80% of the time and also use a Subsee +5 diopter which allows 2:1 magnification. The E-M1 has very good autofocus and decent continuous focus with tracking that I am starting to appreciate in some situations. The 60mm macro lens is super sharp. The m4/3 system with its 2x crop factor has obvious advantages for macro photography (and for wildlife, which I also do). The size and weight of the body and lenses are of course more travel friendly , but to be honest not a game changer compared to full frame once you pack the housing, strobes, batteries etc. For a benchmark, everything I need fits in a Pelican 1600.
I have not used a full frame kit but I dive with people who do, and I believe that the IQ of both ecosystems is comparable. I will say that I have seen images from full frame systems that I don't think I could have made with my setup. This holds especially for wide angle. But I feel no compelling reason to change.