RX100 V advantages over G7X II are:
Hybrid phase- and contrast-detection autofocus vs contrast-detection only - this is a big one, as PDAF lets the camera focus directly on the needed distance, rather than go through the lenses focal distance range until it finds the focus, sometimes several times, allowing for much quicker focusing times. Dedicated PDAF arrays are the advantage that SLRs had over compacts and mirrorless cameras for many years.
4K video vs 1080p
24fps bursts vs 8fps - less important if you use strobes, but very useful if you shoot in ambient light or with LEDs
Slow-motion video capability up to 960fps at 1080p
Pop-up viewfinder - not accessible underwater, but very useful on land
G7X II advantages over RX100 V are:
Slightly better battery life
Better manual white balance for video (Sony tops out at 9900K)
Better macro performance at 100mm EFL vs Sony's 70mm EFL
As for housings, the Meikon housing for RX100 series is rated to 60m, if that has any importance to you, and the lens ring control knob is placed conveniently on the left side of the housing, but there is no rear dial control knob. The G7X II housing is rated to 40m, and has access to all the controls including the rear dial, but, on the downside, the lens ring control knob is placed on the front of the housing, underneath and to the right of the lens port - a much less accessible location. This means that with the RX100 housing, you can shoot with your right hand, and control shutter/aperture/focus with your left (depending on what function you assign to the lens ring), while on G7X, all the controls are on the right side of the housing and you have to move your hand between them, including reaching around the housing (and likely tray handles) to the front.
For accessories, you want a tray (if you get dual strobes or lights) or at least a handle, a spring coil lanyard to attach the camera to your BCD so that it doesn't float away, a neoprene jacket to protect the housing, powerful LED lights if you want to shoot video and/or strobes for still photos, arms and clamps for those lights, a wet diopter for macro (neither camera can focus close at its maximum focal length; the quoted 5cm is at the wide end only) and a wet fisheye for wide-angle (24mm, which becomes about 30mm behind a flat port, is not that wide), adapters to attach and stow those wet lenses, floats (either foam blocks or carbon fiber float arms) to make your rig neutrally buoyant, a bag to transport it all in, spare batteries, chargers, memory cards... it's a rabbit hole down which your money goes, never to be seen again.
