I find it hard to believe that a lot of people find it easy to skin rabbits but don't know how to skin a fish. After all in a survival situation fish will tend to be one of the most reliable sources of food we can get..
Start with your fillet, whatever fish that maybe
Place your knife in between the flesh and skin near the tail
Keeping your knife at a very shallow angle wiggle the fish towards you, holding the knife in place and the flesh will start to lift from the skin
Again using the skin as the lever work the fish towards you keeping the knife at a constant angle
And here's the bonless skinless fillet skin side up to show you the result, the skin can even be dried under the right conditions to make a "leather" that small pouches can be made from..