Fishing
Fishing for Pokemon is a little like fishing in real life, but you capture Water-type Pokemon instead of fish. Many Water-type Pokemon can be found by using the Hidden Machine move Surf and swimming around in any body of water, much like wandering through tall grass. However, not all Water Pokemon can be captured this way and instead must be caught using a fishing rod.
How to fish
Stand in front of any body of water. Lakes, streams and the ocean can all be fished. Open your backpack. Go to the Key Items pocket and select the fishing rod. Your character will pull the rod out and begin to fish. Press the "A" button when the game alerts you that something has been caught. Do not tap the button repeatedly. Only press "A" when prompted, which may take several presses. The Pokemon will engage you in battle once reeled in and can be caught by fighting it to low Health and throwing Pokeballs. You can also assign a shortcut to the fishing rod by selecting "Register" instead of "Use" from the Key Items menu. Press the "Select" button to automatically fish when standing in front of water.
Super Rod
Fishing rods in "Pokemon Sapphire" come in three types: the Old Rod, the Good Rod and the Super Rod. Each new fishing rod allows more difficult Pokemon to be captured and certain Pokemon are impossible to catch without a particular rod even if you are fishing in the right location. The Super Rod is the ultimate fishing rod and is capable of catching the highest level Water-type Pokemon in the game. This rod also has the added bonus of filtering out the lowest level Pokemon, like the annoyingly common Magikarp.
Super Rod Exclusive Pokemon
Sharpedo is one of the most common Super Rod exclusive Pokemon and can be found in most bodies of water. For something rarer, travel to Sootopolis City, the city in the center of a sunken Volcano. The lake in the center of the city is the only location where wild Gyrados can be found, which can be caught to avoid having to level up the notoriously useless Magikarp. The Super Rod is also the only rod capable of capturing Horsea (Routes 132-134) and Corsola (Ever Grande City). For something truly rare, travel to Route 119 to search for the elusive Feebas, a Pokemon that can only be found in a six-square area that moves randomly whenever the "trendy phrase" changes in Dewford City. The Super Rod will prevent any Pokemon other than Carvanha and Feebas from biting, making it easier to find the Feebas spot.