6D for underwater

I have a feeling you'll be disappointed. DSLR housings cost a lot more than $500, and on top of that you'll need an extension port and a dome.

I don't know what prices are for Canon DLSR's, but my Nikon D810 (also full-frame DSLR) housing cost me as follows:

1. Housing (Nauticam, Ikelite is a lot cheaper): $3400

2. Vacuum leak detector option: $220

3. 230mm (very big) dome port for 16-35 lens: $1800

4. Extension tube for dome port: $400

AND it's really big, probably a lot bigger than what you would be looking for. It's bulky at best for packing and travel, and for snorkeling I think it's just too big to be appropriate.

Smaller domes don't give as good a corner performance, and my corners aren't great, they aren't even good without stopping down past F8.

Smaller domes work well with fisheye lenses, and fisheye lenses are popular underwater.

What lens are going to use underwater? The choices are generally limited as they have to have the right characteristics to work well in a dome port underwater, and they have to fit the ports/extensions chosen. In order to zoom, you need a focus gear that attaches to the lens.

Taking a DSLR underwater isn't really a beginner's game. In the past I would recommend a nice Canon point-n-shoot with matching Canon housing. Used to be you could get the camera and the housing for under $500 or so, like my Canon s120, my s95, my sd870is, and my sd630. Trouble is, the whole point-n-shoot class of cameras has been killed off by cell phones.

Now I'd consider this: https://www.bluewaterphotostore.com/kraken-smart-phone-housing?gclid=EAIaIQobChMI6fGL1ejF4AIV1Bx9Ch2luAG3EAQYAyABEgJnC_D_BwE

That's a universal housing for a cell phone.   The controls work via bluetooth from housing to camera. $299.