What can you catch with the Super Rod in Pokemon Pearl?

Gyarados, Whiscash and Feebas can be caught in the underground lake in Mt. Coronet.

Be warned though that Feebas can only be caught in 4 squares, and the 4 squares are rechosen at random every day.

You can catch Wailmer and Wailord in Route 230, near the fight area.

You can catch Luvdisc and Octillery in the water near Pokemon league (just fly there)

You can catch Corphish, Whiscash, and Crawdaunt in Celestic town in the pond.

You can catch Lumineon, Clamperl on route 219 and 221, Chinchou on route 220, and Octillery and Sharpedo on route 222.