Well, I have the Panasonic LX-7 and it too suffer from the limitation of WA lens choices. The Nauticam Wet Mate helped a bit but is not truly wide angle.
I'm in the process of trying something: Using a land wide angle for underwater use. This is what I just purchased for about $90:
If it works, it will be applicable to the Canon G series. It has 140 degree view on land, I expect it to get narrow under water, but still should be wider than the Nauticam Wet mate. The Opteka rear element is big at almost 67mm. This thing is quite heavy. The only question is how is its image/sharpness quality, and how it will hold up under water. For less than $100, I pulled the plug and went for it. I won't be able to go to the pool to try the Opteka lens until mid-March. I'm in the cold Midwest Kansas here getting in and out of pool and car ride is not fun!
BTW, I did consider the Opteka 0.3x lens just in case if the angle get more narrow once it's in the water, and it's cheaper, but the biggest they make is 58mm, so I avoided that.