Questions about Canon PowerShot G5 X

The G5X does not allow you to attach a visor or filter in the usual way.

This camera has a 1 inch sensor and thus a cropfactor of 2.7. 8.8mm then corresponds to just under 24mm at full frame. The sensor size is relevant. Whether SLR or mirrorless or compact is irrelevant. What do you mean by 10mm? With which camera? APS-C SLR / mirrorless? Full format? 10mm on APS-C then corresponds to 16mm KB for Canon and 15mm for the others. There are only a few 10mm lenses for 35mm cameras on the market. E.g. from Voigtländer or Laowa. There are also 12-24mm lenses for 35mm cameras. That is already very wide. Compact cameras with such ultra-wide-angle lenses do not exist on the market. The most compact way is with a mirrorless APS-C camera and a 10-18mm lens (or something similar depending on the manufacturer).

No, Canon does not offer an underwater housing for the G5 X. The G7 X does. The G7 X has one.

No, there is no connection for an external microphone! The camera only has two built-in microphones (to the left and right of the viewfinder attachment), which are used when filming.

Not to my knowledge, but check the Canon website!

Cullmann Ultralight Pro Compact 400: outer compartment with Velcro for spare battery, inner compartment for spare card, good padding. The latter makes the bag a bit bigger than I wanted, but protects the camera well on my many trips.

Not an easy question. It depends on the purpose. The Canon G5x has a bit more zoom and focal length - but is slower in continuous shooting with 6 vs. 10 frames/sec. and has the lower resolution in videos but more fps.... Both have a built-in flash, but only the g5x has the option of a flash shoe. If photos are more important and Full HD videos are enough, I would tend towards the Canon G5x - if high-resolution videos are required, I would go for the Sony RX100 IIII.