To get the super rod you have to do the following
A.) See all 210 Pokemon in the sinnoh pokedex (not catch...just see them)
B.) Visit Prof. Rowan and he will update your pokedex for you.
C.) Go To snowpoint city
D.) Get on the ship to the Battle Zone
E.) Once you have got off the ship go north straight from there and you should now be able to go further than you could before.
F.) Talk to the fishing guy there