There's only one way to fish.. make sure you don't look at vanilla P4 guides, especially in terms of fishing.
If you've already got the fishing rod and you've unlocked the beach, go there and try to fish, then talk to the old guy at Samegawa. Catch Inaba Jewel Beetles, go fish. Keep reeling in the bait (don't wait until a fish bites) until you start getting large splashes instead of small bobs, that's the sign of the guardian. --- Official [character] of the [game] board. "We'll go get you some mashed potatoes." - Naoto
Start the rank 10 Hermit S. Link, go to the beach (3 neighborhood bike rides and 3 long distance to unlock), attempt to fish, then go to talk to the old guy at the Samegawa Riverbank using and keep talking to him util you've asked everything he has available. Then at night go to the shrine and catch bugs, you need to get some Inaba Jewel Beetles. Go fishing at the Riverbank using the IJB (rainy days work best) and keep recasting until you get a large splash. Reel it in (much easier if you've read Expert Fishing and got the quick-reel).