I’m looking at recipes to grill fish, particularly recipes involving a lemony marinade, and several recipes warn not to marinate the fish too long as the lemon juice will begin to “cook” the fish.
Chemically, how and why does this happen, and practically, how does it manifest (is it something I can see happen, does it affect the taste, etc.)? And if the lemon juice does begin to cook the fish, does that render the fish unusable?
Essentially, acids lower the pH of the food (fish, in this case) which lowers the negative charges on the protein molecules, which encourages coagulation. So, really, it’s a slow version of cooking, rather than heat causing a quick and forceful coagulation. You’ll be able to see it–it takes a little time, but you’ll notice some spots where the flesh is going to look a little firm and “cooked”. It won’t be unusable, but you could have some dryness to the fish.
Honestly, I’m fonder of just seasoning the fish with a little salt & pepper, then dousing it with the lemon juice when it comes off the grill. 
Mind you, lemon/lime juice is what “cooks” ceviche, too.
Vodka-lime salmon ceviche is goooooooooood.
Yes, ceviche. “Raw” fish in the sense that it’s only cooked by the acidity of the lemon or lime juice that is used.
Mmmmmm…
Related question: Does the acid “cooking” also kill off bacteria or other upleasantries? In other words, assuming it’s not pure “sushi-grade” fish to begin with, is the stuff safe to eat if not exposed to any heat?
I recently found while exploring the icy depths of my freezer a package of “krab”, which I think is also called surimi.
I am thinking about using this to make ceviche. Is this a good idea? Can anyone provide a good recipe for ceviche using this bogus crab? Lemon juice I have, and I guess I need onions and some kind of chiles.
It’s the same principal in that acid affects the pH balance, which, as long as the seafood is fresh, being stored in properly refrigerated space, not being handled by Typhoid Mary, etc. should be safe to eat. Not saying that it will kill off every single little bug, but it’ll be safe to eat. My in-laws, who hail from South America, make ceviche regularly, and never use sushi-grade seafood–and I’m still quite alive. 
Part of the problem with using krab is that it’s already cooked, and part of the making of ceviche is the hours-long marinating in the lemon juice that not only cooks, but helps flavor the dish. You could make a kind of ceviche-ish kind of salad that does not require the marinating, but just with everything fresh chopped and served as is. Ceviche in general has onions (red or white), lemon/lime juice, tomatoes, chiles (jalapeno or serrano) and cilantro, and sometimes you’ll see cucumber and/or avocado added.
To be honest, I’d stick with using the krab for a more traditional salad, and go buy some fresh shrimp or scallops or real crab for the ceviche.