You can only use fishing nets in deep water. You can use the fishing nets regardless of your fishing skill. Fishing skill increases your chances to pull up Scalis using a fabled fishing net. According to developers, at GM fishing, your chance to pull up Scalis using a fabled fishing net is 25%.
I first checked uoguide and then tested the things above to answer your post. Also, I just pulled up Scalis with 0.0 Fishing and using a Gargoyle char (so no Jack of all Trades Fishing skill bonus either) at my very first try. It is now in New Magincia docks in Europa if you are interested. It doesn't mean that it will spawn at your first try too, but it means there is still a good chance for you to spawn it without the fishing skill.
Edit: Two good spots for Scalis is the Buc's docks and the New Magincia docs, both of which are long enough to reach deep water.