Expect to pay about $80-$100 for an adaptor for the canon housing. A dedicated u/w WA wet lens will then cost another $400+ at least. Not sure how much advantage the WA lens will give you as the G series of cameras usually will have some vignetting when used together with a WA wet lens. Zoom in and you might lose the 'advantage' the WA lens will give you.
Some food for thought.