Tell you what, mate, I’ll give you the main pre-prep item for now, as the writeup on that’s about finished:

MARINADE: Prepare it in advance. For general purposes*, that would involve squeezing a bunch of (key) limes (and maybe a lemon or two, to taste) into a good-sized container, then adding thinly-sliced red onions and thinly-sliced fresh peppers, as hot as you can stand. For me that’s habaneros, for someone else that might be jalapeños, etc.

• Let that sit in the fridge at least a day to completely saturate the onion and chilis, as well as merge their flavors.

• In the meantime, try to find a local place that has tasty cilantro. I suspect growing conditions are the main factor in that, and I’ve found that some cilantro is almost flavorless, while some has more of that classic ‘zest.’ Note that if you’re a sufferer of that cilantro-tastes-like-metal gene, then you’ll have to skip it, but possibly you might enjoy a substitute; perhaps fresh basil or mint.
• Regardless of what you do next, you’ve just created a batch of a lovely garnish that you can add to loads of things… sort of a hot sauerkraut alternative, but fresher-tasting, and without the salt!

Anyway, that’s what to marinate the fish in. Generally the thinner-sliced, the less marinade time needed.

I have more to add about selecting seafood and food safety, but the marinade is of course, the base of the whole recipe.