you go to water and use a old, good or super rod in key items!
You can fish with a super rod
You have to fish at Sootopolis City.
You have to fish in Dragonspiral Tower
you fish or surf and throw a pokeball at it, pretty straight forward
Just use any rod on any body of water (as long as it's not a puddle!), and you'll catch water pokemon.
At route 218 get out your super rod and fish.
goldeen is very common, and you can also catch finneon and psyduck. But it varies with where you fish. You can get all kinds of Pokemon, so try everywhere!
catch a water pokemon with a rod and fish until a pokemon comes. then teach the water pokemon surf.
Go to Mt. Coronet and on a certain floor you can fish for it!
Fish for it outside of iron island with a super rod ;)
You can find it if you fish in the water near the Day-care.:)
It's called Relicanth, it's a rock fish, you catch it at the bottom of the ocean. -Catsura