Why yes, I do live near Montreal. Yes, I have been to fish mongers here, but I can't stay in them very long because of the smell. I lived in Copenhagen for five years. In Copenhagen you can get fish caught the day or day before you buy it. I'm very picky about the freshness of the fish I eat. I really don't want the fish to smell of harbour. The fish mongers I went to in Copenhagen certainly smelled of fish, but not harbour. Fish smell, to me, is a strong aroma, not unpleasant. I would rather have the slight change in texture and possibly loss of brightness of the flavour that comes with freezing, than smell harbour. Maybe I'm just especially sensitive to that smell.Are you close to Montreal? Have you never been to a fishmonger. Anyway there's plenty of fresh fish (never frozen) to be had, it's more expensive. In a restaurant setting where a reputation for freshness and made from scratch the last thing you need is to have your brain fall out of your head and serve previously frozen ceviche.
Personally I wouldn't bother making ceviche if the fish had been frozen for a few reasons. First, the texture is not the same and after adding an acid like lime it's really noticeable as the fish gets tougher. Frozen fish, even flash frozen the ice crystals that form within their cell structures loose this water making the fish dryer, tougher and less malleable. Also visually, fresh fish is bright and translucent looking compared to previously frozen and a side by side comparison it's pretty obvious. Although I'm always surprised how many "fine dining" places are using frozen. I always ask the waiter if the ceviche is from fresh fish and if they weren't sure, to then go ask. This of course is just my opinion.
I find that frozen salmon works very well for gravad laks. With no previous experience with ceviche, I guess I'll find out when I try to make some.