Guide by bernamsa at 21 Apr 2023. Link below.
https://www.trueachievements.com/a381488/fishing-master-achi...Long not very clear process:
First you need to unlock the beach by taking 3 long rides in your bike.
Try to fish and the game will tell you that you can't with your current rod.
Talk to the old man fisherman by the flood, and ask about fishing at sea, he will tell you that if you catch the lake guardian he will give you his rod.
In order to fish for the lake guardian you need to have the jewel beetle and fish on a rainy day.
Show the lake guardian to the old man will give you his rod and talk about the sea guardian.
On a rainy day you need to fish at the beach with a genji beetle, if you see big waves coming off your bait it means you have spawned the sea guardian (its not a guarantee spawn so you have to save scum, a lot).
Catching him is hard, the fishing book doesn't make it easier, btw, my suggestion is to use the d-pad to maneuver and mash A when you have a blue opening instead of holding the button, will take around 1-3 minutes to reel.