Hi!
This is a slightly tricky question to answer as, in many ways, you are comparing apples, oranges and, err, bananas....
GoPros are terrific for video. From what I hear, pretty hopeless for underwater still shots. But they have the advantage that they are relatively inexpensive, editing software is free, you don't need a housing and you can get away without lights if you stay shallow. But for serious underwater stills, not so much.
Canon G7xs, like most P/Ss suffer from shutter lag. If you are trying to take serious fish portraits this can be tricky as the rascals tend to move before the camera has finished composing and taking the shot. I'm not sure what battery life is on the G7x but this can also be a problem with P/Ss as you might need to remove it from the housing reasonably often to replace or charge the battery. But if you accept the limitations of the camera, you should be able to get some good shots. You may well need a strobe or two. I'm sure others on the forum can tell you more about the pros and cons of the G7x.
The Canon EOS is a different animal again. You will/should get better shots by the very nature of an SLR. Shutter lag is not an issue. You will need strobes. Yes, buying a housing is a wounder (welcome to our world!). And you may well need different ports for different lenses (think of the difference between macro and wide angle).
So, as I say, apples, oranges and other bits of fruit.
I'd suggest having a read around a bit - there are some very good info sheets on Reef Photo (http://reefphoto.com/shop/) website and Cameras Underwater. (http://www.camerasunderwater.co.uk) and see if you can have a look at a copy of Martin Edge's excellent book, Underwater Photographer. These will give you a good idea of what equipment is needed to produce what kind of image.
In terms of cost, GoPro would be the least costly, then the G7x and then the EOS the most costly. The problem can be that if you decide to embark on the middle option, your Canon G7x, you might find quite quickly that you can't get the kind of images you are seeking - and you have spent a lot of money and only a small amount of what you have bought could possibly be recycled into an upgrade to housing the EOS. The strobes and arms, for example, could well be transferable between those two systems. But the costs of upgrade and the loss on your original purchase could well be significant. And then if you splash out (bad pun) big time on housing the EOS you might find it's all too much of a faff and not worth effort. It happens!
So, coming almost full circle, have a good read around and try and get an idea of the equipment involved in each and the likely results that you can get with practice. This might help you reach a clearer view. Then (re)mortgage your house, sell a kidney and your first born - and you're one of us 
Hope this helps a bit. Good luck with exploring!