here are some pointers about fishing:
you can fish in any body of water, as long as your character can float in it. you'll want to fish in special sections of water though. for instance, when you go to the shoreline, you'd probably see a shiny place in the water. upon hovering ur mouse over it, the sign would probably read the name of a type of fish. when you fish in that particular school of fish, you will catch that type of fish.
fishing doesnt mean you'll catch fish every go. occasionally you'd pick up armor, crates, and other goods. to make fishing go more smoothly, purchase fishing rod add-ons, such as nightcrawlers or baubles. these items increase your fishing skill, so you'll be more successful in catching fish. to use one of the items, right click on it in your inventory, then use it on your fishing rod, and fish away!