In dewford Island and Mossdeep city forgot about good rod though..
the fisherman in right of mauville in the other side in the mosdeep or sootopolis (i'm not sure ehh)
if you want fishing pole in ruby you need to beat elite 4 then fly home and you mum will say that you dad called and want you to come to his gym fast then he say hes friend Pokemon is stuck in a cave then just go into the cave (don't know the name) and search around and you will see a Pokemon on the floor press A on it so it not so hard
in the somewhat tall grass. or when you are fishing with a fishing pole. or when you run into a Pokemon when surfing on water.
With a fishing rod that you can obtain in Dewford.
You can obtain Horsea by fishing on Routes 132, 133, and 134 in Pokemon Ruby.
at jubilife or enterna city
magikarp
you get it in the house on silence bridge
just surf on the ocean..
yes you do need a fishing pole to catch mitotic in Pokemon black but you cant catch that Pokemon you have to catch the first evolvation and then you train it until in evolve into a mitotic
go to the lake of rage
Heck if I know!! Haha