Hi Nick. I’d be a little wary of the G7X if I were you, it does not behave like most other compacts so needs some consideration. If you are thinking of going wide angle, the G7X is particular in that its lens is shorter when at its widest. The result of this is that if you screw a wide angle lens to the front of the standard port, you are left with a large air gap between the camera lens and the port glass with a result that the corners of the image become very soft and out of focus with heavy vignetting around the borders of the image. The alternative is to zoom in which then means you lose much of the benefit of the wide angle lens.
Nauticam have overcome this issue by offering interchangeable ports for this camera, a shorter port for wide angle and a longer port for macro. When you have the short port installed, you cannot zoom in beyond about 40mm as the lens extends and hits the glass. This does mean that you lose flexibility that you would normally expect with a compact camera but you do end up with good image quality for both wide angle and macro. As far as I’m aware, other manufacturers do not offer a similar solution.
We have sent a setup to a French magazine, Chercheurs d’Eau, who are currently running tests with the Nauticam, Isotta, Recsea and Ikelite housings for G7X with a variety of wide angle lenses so this will be worth watching.
If you are looking for flexibility to change from wide to macro underwater, I’d still personally think the Sony RX100 Mk2 is the winning choice for underwater. This camera though does not perform well underwater for white balancing and the flash recycle time tends to be longer than the Canon.