I usually dive with a Nikon DSLR camera in a "big" Subal housing. Recently a friend lent me a Canon G7x (in Canon WP-DC54 housing). It was a pleasure to test this compact camera. I enjoyed to use it underwater because it is tiny in size, has a good macro capability (minimum distance 2" / 5 cm) and an interesting focal range with fast aperture (24-100mm, F1,8-2,8). It produces high quality images (1" sensor size, 20 megapixels, shoots in Raw). I didn't use an external strobe, only its internal flash which can be set to TTL or manual power level. It would be better to use one or two external strobes for better light control and more power (brighter or broader coverage). I liked the small volume of the housing, it allows to reach subjects that can not be photographied with a big housing and cumbersome strobes.
The only two things that bother me are:
1- The shutter lag (like any other compact camera!)
Even though it has a pretty fast shutter response, there is a lag. For example, it is not easy to take a photo of a very active fish which suddenly changes direction.
2- The AF performance / unreliable AF system
Autofocus is quite slow and the focus accuracy is sometimes bad, especially in low contrast situations (unable to lock focus).
The Canon G7 X is an exciting camera with strengths that make it a great choice for underwater photographers who don't want to dive with huge, heavy and expensive photographic equipment.