Buy yourself a Fastwater Rod and a Sabiki lure. it'll run you around 3k. Once you have that rod/lure combo stick to the port and fish. You'll never break the rod and you can get up to at least lvl 8-10 with that combo. Also make sure to get the fishing imagery cast on you from the fishing guild that should be right next to the spot you're fishing at. With the Sabiki you have a chance to catch up to 3 fish in one cast with it. Makes up for all the times you lose your catch.

Also the best day to fish is on Watersday. Also the best time to fish is during a New Moon or Full Moon as the moon effects the tides. I believe there's specific times of day too that are optimum times to fish.

Tips from fishing guild:
CrayfishFreshwater Pond/SwampMoat Carp Bait
Moat CarpMoats?Ball of Insect Paste
Bastore SardineSouthern SeasLugworm
Observed Catches:
CrayfishSmall WormFresh Water
Bastore SardineLugwormSaltwater
QuusLugwormSaltwater
Yellow GlobeSabiki RigSaltwater