Hi!
I'm going to add some more information as I'm playing, so stay tuned!
I think I have finished it. The fishing mechanic in this game feels unfinished and you don't really have control over the quality of the fish, just how fast you will be able to reel them in, which, to me, is a bit of a shame. Either way, i tried my best with this guide!

In this guide you can find the following information:

• General tips and tricks
  
• Fish, fishing locations and seasons/conditions
  
• Foragables on and near water

• Weather, season, or time has no effect on your catches.
  
• You can buy lures and hooks at the fishing shop. They help with catching specific fish or make the mini-game a little easier... But I prefer fishing without them, I just adjusted the difficulty.
  
• Find the mini-game too hard? Adjust the difficulty in the Settings menu (Settings > Gameplay > Fishing mini-game difficulty).
  
• If you time it just right, you can get a perfect throw... But I don't think it has much effect on the quality of your catch. I believe fish just bite faster.
  
• Tame 3 Snorckos and have them follow you around—they have a passive ability that makes fish bite faster.
  
• Sometimes you can find islands with a little piece of land (an... island on the island?) in the water. These are perfect for fishing; usually, they are small enough that there won't be any spirits spawning, so you can fish in peace. Once you get the trinket from the Water Temple, you can go fishing on these islands, even if the island is a higher level than you!
  
• You can buy upgrades for your Fishing Rod at the fishing shop by clearing the 3 objectives of the Mythical Fish quest. Every objective you clear unlocks one new upgrade:
  The Reinforced Fishing Rod: Fish will bite faster.
  The Greater Fishing Rod: Casts further, and fish will bite faster.
  The Moonstone Fishing Rod: Casts further, reels in fish quicker, and fish will bite faster.