Fishing is a great way to catch Water-type Pokémon in Brilliant Diamond and Shining Pearl, but you might need some special rods to catch the rare Pokémon. At first, you can get the Old Rod, but will soon need to upgrade it. You can get the Old Rod by traveling west from Oreburgh City to Route 218. There is a man on this route who will give you the Old Rod if you speak with him.
To get the Good Rod, players will need to make their way to Route 209 to the east of Hearthome City. Players will end up arriving in a location with water and lots of wooden piers. Speak to a fisherman NPC here to get the Good Rod.
The Super Rod is far more difficult to get than the other two. It can only be obtained after defeating the Elite Four. Make your way to the northeast of the map and visit the Fight Area and you will find yet another fisherman. Speaking with him will get you the Super Rod.
Fish rods actually prove very useful for increasing your array of options in Battles, and finishing the PokéDex, which you will need to do if you want to move on to the national Dex and all the more secretive Pokémon that are hidden throughout the game.