Special fishes (large fish, crab, lobster) used for delectable food which buffs players stam medit and hit regen.
Specials fishing points where you can spawn monsters rarely mib (sea treasure map) drop in loot.
Mibs can be done with only fishing also need to have crew or skills for dealing treasure monsters.
Treasures from mibs sometimes good loots sometimes garbage sometimes nothing.
Also you can do society jobs with fishing but usually it s really hard.
Finally if you like playing with fishing do it but if you think about get richy there are better ways