Would Canons S110, S120 not have the same vignetting problem as G16? They both have a 5.2-26 mm lens vs a 6.1-30.5 lens on the G15/G16... Is that enough of a difference?
As the S series only have a 4x zoom to 100 they work fine with traditional wet lenses when step zoomed to 28mm or 35mm
The G series with their 5x always had issues, only option a recsea housing selling at $1250 for the G11/12 with a dry wet lens that had fisheye, expensive set up