you have to beat the elite four and have seen all 150 pokemon in the game. this is possible, you just have to make sure youve battled all the trainers with unique monsters. then you have to get the national dex from the proffessor in celistic town or whatever its called, basically the place with the lab.
on the battle island place there is a north path just as you get off the boat. this area will be accessable once youve gotten the national dex. there is a fisherman stood just outside the entrance to the north who will give you the super rod.