if you are snorkeling (primarily) with occasional dives. I would really consider NOT buying that housing for your canon. Instead buy a compact (used you can get cheaper) with a wide angle lens.
Remember the number one rule for underwater photography. Get Close. A wide angle lens will let you get close to your subjects and get it all in the image. You can also shoot WA more easily with available light. Something you cannot do easily, if at all, with Macro or "normal" focal lengths
I think if you look for a wide angle compact system, you will have a lot of fun and will get more images that you will be happy with.
Although your canon has better quality than a compact, it will not take great photos with a normal lens (even at 18mm behind a flat port) with a lot of distance between you and the subject. Also, you can get spectacular photos with a compact, they are very capable today.
also a compact camera will be easier to snorkel/dive with if you aren't used to it.
A quick search in the classifieds showed this guys is selling something that would suit you very well (no affiliation)
In my opinion, i would highly consider something like this.
- tdpriest likes this