You need to search your soul for your expections into this form of photography.
All cameras no matter how good are capable of taking snaps, however to take images that sometimes one desires it takes a considerable expense, time and knowledge to master the art.
If your looking for just family memories that involve a splash I'm sure both systems will serve you well. If your looking to dive a with system then you need to consider which type of artificial "sun" you'll be taking underwater with you. Light is essential to replace the wave length of light "RED" primarily that is lost as you dive deeper down the water column.
You don't always need light, as you could use colour filters or adjust using a software program to a certain extent.
The Hobby/Profession tends to be an expanding one, so be prepared for this ! Once captivated by UWP you tend to strive for better and better images. To capture those images you need an assortment of light focusing devices "lenses" and something to paint light onto your subject "flash"
It's no use rushing out and getting the best of the best. If a poor eye and brain is behind the viewfinder, only snaps will be captured. With effort, knowledge, equipment, perseverance and time you can of course produce something that is outstanding.
I'd more than likely go with your compact system to start with and save that D750 for something it desire's like a proper housing. It will also cost you less to replace should the worst happen. If your going to go through the expense of getting a new housing for a Canon 100D I'd advise you not to. The camera might not be able to grow with your needs as you progress.
The expense of the housing isn't all that different in comparison to a new housing for your D750 from Ikelite. So I'd just bite the bullet and get a D750 housing if you really want to do this at considerable expense. The Nikon D750 is a very capable machine and I'm sure will serve you well should your aims expand.
Also and one big also, all of what you see is not all of what has been captured by the camera these days. The art of photography since the arrival of digital has become a multidisciplinary art, that of image taker and image manipulator.
Wish you the best of luck choosing what to go with.
Edited by craig nelson, 22 June 2017 - 12:02 PM.