-- Last Updated: Feb-17-13 4:25 PM EST --
Gopro 3 has 12 MP in resolution. The lens and software combo does favor single width, wide angle to very wide angle, which some won't like, I realize. Fogging is cured with cheap anti=fog devices. The housing is very rugged, goes down deep and the dive housings go deeper. The basic units all come with the waterproof housing as part of the package, so you don't pay extra unless you want one specifically for diving deep.
Your G15 is a $500 camera requiring another $300 underwater housing and has a 12.5 MP resolution. So at twice the price you can have a great zoom, but it's bigger and won't work as well for video nor for places where you need small light camera.
If you understand the Gopro's intended major use - video - and its secondary use - stills - you have a very versatile camera system to work with. If you want a zooming, truly waterproof still camera that might shoot short videos, then you don't go the Gopro route, something like the Canon will do better.
Just depends upon the intented use, really.