I'm not fully abreast of all the Canon options out there but "do-everything" systems sacrifice quality.
Generally no, it is not an option for SLR rigs. Nor do we really want it to be.
Not only do the shorter range SLR lenses generally offer much higher quality, they force you to 'focus' as a photographer. If you're shooting wide, you wave hello to the pygmy seahorse and then go find some cool wide shots.
I'd recommend to any non-professional to go mirrorless for UW. SLR housings are a real commitment, you become beholden to some really bulky and heavy crap. The quality gap between mirrorless and slr has shrunk significantly but the physical difference in UW rigs is huge. A high end mirrorless camera and lens will outperform a low end slr and lens. Lenses are more important than cameras.
Portability can make a huge difference in many shooting scenarios too, think about the gopro revolution for action footage.
I'm definitely of the belief, given today's camera market, that if you don't know you want an slr, go mirrorless.
Its likely not a sensor issue. Generally this is created by some form of 'fogginess' somewhere within the light path. Beyond the obvious of cleanliness it could be:
1. Within the lens from mold, but usually only excessive mold.
2. Outside the lens within the port from condensation. This can be from an uncovered port sitting in the sun or just a warm camera rig taken into colder water. It can take a long time to dissipate on its own.
3. Due to the UW visibility especially with bright areas.
Regarding #3 - this is something I've seen regularly with multiple camera rigs shooting humpback whales in open water in midday sun. Their white pectoral fins are so much brighter than anything else and surrounded by these halos. Its better controlled on newer cameras but many times unavoidable. It can happen in good visibility but is far worse in poor visibility.
I've also personally seen this with scenarios 1 & 2.
Beyond that my port even has what some on here have described as mineral deposits on the outside from the water over years of use and imperfect care. Its visible on the port when looking at certain angles and light. Pretty sure it contributes. Planning to try and polish it off.