We love marinade fish done Island style this was taught to me by two different people, one is Tongan the other rarotongan, but to me its very similiar and all the same...but this is my combination...

Fresh and only fresh fish fillets never from frozen...

With a super sharp knife slice fillets into thin pieces, sliced at 45 deg angles and along grain areas of fish, hard to explain via typing but sashimi cuts are a speciality and so is raw fish, you have to slice and cut by dragging the knife backwards not push the knife or chop and squish the fish with the blade and nor go straight up and down, use the angles to achieve the tenderest of pieces, very important for that high class melt in your mouth dish... very worth the effort with a good knife and technique.

Combine Lemon and Lime juice about 2:1 mixture and pour over the fish in a long low marinade dish then grate a large Onion on the fine grate side of the grater. (Hopefully the pic will show) This is a tough and slow job and what you achieve is juice and very fine pith from the onion and you scrape that of both sides of the grater and off your choppping board into the fish dish and after about four times with a big onion you give that a huge and slow mix and stir and allow that lot to soak for about 3-4 hours, stirring every hour or more.

In the meantime and whilst that is settling and on the soak, chop into sort of diced sized pieces of veges which adds a salad component to your fish dish... I use tomatoes, capsicum red, yellow and orange, spring onions, cucumber, ... and so I add the veg after the 3-4 hour marinade and then coconut cream and a big stir and then chill it...in the Islands they would add much more and better from the likes of PawPaws, PineApples, mangoes, melons and so on... so that the entire dish was rich in protein, fruit and veg...

If you don't have any coconut cream then just used standard cows cream, tastes just as good, maybe a little more fattening 

Nevertheless two great raw fish dishes highly recommended