I don't own a Canon T3i but have owned a GoPro. The Canon DSLR is a superior camera and sensor. The GoPro is has a much smaller sensor thus not on the same level IQ wise.
Canon pros:
Excellent photo IQ
Better low light capabilities
Better video IQ
More options to adapt to different scenes
Zoom
Canon cons:
Large and heavy
Can require an assortment of lenses
Limited to lens on camera once in housing UW
Quality housing can be very expensive
GoPro pros:
Compact and light
Simple operation
Good wide angle FOV
Above average video IQ with sufficient light
GoPro cons:
Poor photo IQ
Subpar low-light video IQ (lots of image noise at high ISO)
Not so good battery life
No zoom
Fixed focus
Lack of advanced camera features
If you looking for good topside photo performance and variable zoom (FOV) the GoPro probably won't perform to your liking. The GoPro has limited capability but as a light, simple and compact UW video tool for wide angle video in decent ambient light, it can be quite a pleasure to work with and provide great results.
The DSLR can do just about everything the GoPro can and better but at a cost. Bulkier setup UW and the need to understand and fiddle with the camera functions to ensure proper IQ.
Unless your really hard up on UW photos, I'd stick with the Canon topside for photos and video, and settle with a GoPro for easy UW video minus the UW photo.
IF you DO want UW pics, get a decent housing for the Canon and a strobe, that is a must but get ready to pay more.
Good luck