You can do macro with 5A and diopters, but its relatively short (25.7mm actual, 70mm FF-equivalent) lens won't produce strong magnification even with close-up lenses. Diopters let you get closer than the bare lens would allow by itself, which is what gives you more magnification, but at some point the subject is on your front glass and you physically can't come any closer than that. If you want to do supermacro, i.e. subjects the size of a grain of rice or thereabouts (say, leaf sheep slugs), you will need the longer lens of an M6/M7 as a starting point. However, that longer lens presents housing challenges - you need a long port to accommodate it at full extension, and with a 67mm diameter front glass, it will vignette when the lens retracts into the housing for a wider angle of view, restricting this port to macro-only applications. Nauticam has a port with an elliptical front glass which allows the lens to operate over its full zoom range without vignetting and takes a flip-up adapter for close-up lenses for macro, but it won't work with wide-angle conversion lenses, as they require the lens front element to be close to the glass, and in this case, it ends up being deep inside the port. The bundled wide flat port on the SeaFrogs Salted Line housing also allows full range of zoom, but does not take accessory lenses at all.
Don't forget to also budget for extra batteries, charger (RX100 cameras don't last very long on a single charge, and if you're on a liveaboard, you want your spare batteries charging while you're in the water), vacuum system, tray, arms, clamps, lanyard, and of course strobes. A focus light will help with macro shots as well.
