use a super rod in the route left of mauville.
it evolves into crawdaunt
You evolve corphish at level 36.
Lvl 30
Fish for them on Routes 102 and 117 or in Petalburg City
Get a corphish and level it up to level 20 or so and it will become crawdaunt.
use a fishing rod at the bottom of petalburg town! at the pond at the bottom
you go to route 117 and fish or surf on the ponds until you find one same for sapphire
try surfing around a beach, if you cant do that hope that fishing gets you one, though i doubt that will happen.
Depending on where you use it you can find various pokemon like Carvanha, Corphish, Sharpedo, Feebas, Staryu.
A corphish is better.
Corphish is a Water type pokemon.
Corphish evolves into Crawdaunt at level 30.