I played Pokemon emerald a long time ago, but I think I could still answer this question, the Pokemon you find while fishing will be formatted by each rod separately.
Old Rod : Magikarp, Tentacool, Tentacruel, Goldeen
Good rod: Magikarp Tentacool, Tentacruel, wailmer, Goldeen, Corphish, Barboach, Carvahna, Remoraid
Super rod : Wailmer, Sharpedo, Corphish, Baroboach, Whiscash, Carvahna, Seaking, Remoraid, Octillery, Staryu, Luvdisc, Corsola, Horsea
If this is to hard to read I just wrote a simplified version of it.
You can find Magikarp, tentalcool, wailmer, tentacruel, corsola, luvdisc, corphish, barboach, carvahna, octerillery, staryu, remoraid, whiscash, goldeen, seaking, horsea, and sharpedo while fishing
Anyhow I hoped I helped