How does one remove the "fishy flavor" from seafood?

Fresh seafood shouldn't be overly "fishy". It's generally older seafood that will get a stronger "fishy" smell and flavour.

That said, there are 3 approaches to a "less fishy" result:

1. Absorb/reduce fishiness: you can always try something like soaking it in milk (which you can then save to use for a fishy bechamel when your sister isn't dining with you). You could then rinse it off and cook it however you want. This should help "absorb" some of the fishiness and make it milder.
2. Disguise fishiness: The other angle is to "cover up" the flavour with lots of spicy glaze or garlic/lemon flavours instead.
3. Pick mild seafood: The other thing to try is to pick a more mildly flavoured fish (e.g. a white fish instead of something like tuna or salmon). Avoid oily fish as they tend to have a stronger flavour. Choosing something really fresh also falls into this category.

An old trick from the Indian subcontinent is to rinse it well, dust lightly with salt and powdered turmeric and leave for half an hour or more. You can rinse this off before cooking or leave it on - turmeric is a good source of that very desirable umami taste.

One additional consideration in fishiness is the fat itself. My wife dislikes fishy flavor, and so when she eats salmon (for example), she enjoys the pink parts, but dislikes the gray (which are typically near the skin where much of the fat resides). Most of the 'fishiness' is indeed in that oil - a slight rancidity to the oil, perhaps, but more often simply the oil itself is what is disliked (google "fish oil pills" and you'll see all sorts of comments).

As such, I cook the fish in a manner that does not disperse the oil (so grilling or sautéeing is out - a good poach in vegetable broth works though) and then give her the pinker parts, reserving the fattier, tastier parts for myself and the little ones: as such, we both win (I get better fish, my kids get smarter, and my wife doesn't complain about fishiness!)

I had an "old salt" say that it is the "belly meat" that tastes fishy. When fileting, don't cut all the way down to the belly. Seems to be the secret.

Soak the fish covered in water with salt and sugar for 10-30 min. About 2 tbsp of salt and the same amount of sugar when using a cup of water.

After soaking, rinse the fish, then rub in the same amount of salt and sugar (like a dry rub). You can add some black pepper too, if you like.

Let the fish sit in a bowl until the oil starts coming out of the fish (30-60 min). Pour out the oil and let sit some more. Keep waiting and pouring out the oil until most of the oil is pulled from the fish.

The amount of fishyness you pull from the fish depends on how long you want to keep pouring off the oil. An hour or two is OK for salmon. It might take overnight in the fridge for fishier seafood.

After extracting the oil, rinse the fish very well. Cook or smoke after that.

My mom taught me to soak all seafood in milk to remove the strong fishy flavor. It really works!

Mix vinegar with fresh water then soak fish in it for 5 minutes; rinse and dry it with paper towel before marinating. This method will be able to reduce 50% fishy smell. You should not cook fish if you want to remove fishy smell 100%. Ben

cajun spice, salt, and flour mix coating fried in canola oil works like a charm.

Oily fish, such as bluefish or herring, can be balanced by including acidic ingredients like tomatoes, lemon, or vinegar in the cooking process and/or finishing sauce. Think of it as making a well-balanced vinegrette.

I understand the original question, at least as to how it applies to prawns. There is a word in Indonesia (where seafood always comes to the pan still kicking) -which refers exactly to an unpleasant 'seafloor' or 'bilge' smell -and taste present in some prawns. I've noticed it particularly in some Australian prawns though whether due to age or species I dont know. I haven't tried the suggested milk solution, but an overnight soak in salt does help. As for fish, assuming it's all very fresh of course, different kinds of fish can have vastly different smell and taste so it's important to get to know which is which. A GOOD fish supplier (usually NOT yr corner fish'n'chips shop) should be able to advise.

Cut out the red meat aka bloodline and discard. For fish like a Tuna that has red colored meat, cut away the darker red portions and discard. Soaking in milk also works.